diff --git a/biome.json b/biome.json
index d503df20..2b175287 100644
--- a/biome.json
+++ b/biome.json
@@ -6,6 +6,7 @@
       "packages/**",
       "modules/*/*/pages/**",
       "modules/*/*/components/**",
+      "modules/*/tests-js/**",
       "!host/client_app/modules.generated.ts",
       "!host/client_app/modules.manifest.json",
       "!host/client_app/modules.generated.css",
diff --git a/docs/modules/background_tasks.md b/docs/modules/background_tasks.md
index 0772e693..29c10a9a 100644
--- a/docs/modules/background_tasks.md
+++ b/docs/modules/background_tasks.md
@@ -148,8 +148,8 @@ DB-backed; defaults are in `BackgroundTasksSettings`. Several are marked `requir
 | `stuck_after_seconds` | `300` | heartbeat-staleness threshold |
 | `stuck_sweep_interval_seconds` | `60` | beat cadence for the stuck sweep |
 | `purge_interval_seconds` | `86_400` | beat cadence for old-row purge |
-| `retention_days` | `14` | how long to keep terminal rows |
-| `max_retries` | `3` | informational; tasks define their own policies |
+| `retention_days` | `14` | how long to keep terminal rows; range 1-3650 |
+| `max_retries` | `3` | informational; tasks define their own policies; range 0-100 |
 
 Bootstrap env-var equivalents (`SM_BG_TASKS_*`) only seed pydantic defaults at first boot — once a value lives in the DB, it's authoritative.
 
diff --git a/modules/background_tasks/background_tasks/settings.py b/modules/background_tasks/background_tasks/settings.py
index 63d84a6d..d052ea83 100644
--- a/modules/background_tasks/background_tasks/settings.py
+++ b/modules/background_tasks/background_tasks/settings.py
@@ -63,8 +63,18 @@ class BackgroundTasksSettings(BaseSettings):
     stuck_sweep_interval_seconds: int = DEFAULT_STUCK_SWEEP_INTERVAL_SECONDS
     purge_interval_seconds: int = DEFAULT_PURGE_INTERVAL_SECONDS
 
-    retention_days: int = DEFAULT_RETENTION_DAYS
-    max_retries: int = DEFAULT_MAX_RETRIES
+    retention_days: int = Field(
+        default=DEFAULT_RETENTION_DAYS,
+        ge=1,
+        le=3650,
+        description="Days to keep terminal task execution records (1-3650).",
+    )
+    max_retries: int = Field(
+        default=DEFAULT_MAX_RETRIES,
+        ge=0,
+        le=100,
+        description="Configured retry ceiling; individual tasks define their own policies (0-100).",
+    )
 
     @model_validator(mode="after")
     def _forbid_localhost_broker_in_production(self) -> BackgroundTasksSettings:
diff --git a/modules/background_tasks/tests/test_background_tasks_settings.py b/modules/background_tasks/tests/test_background_tasks_settings.py
new file mode 100644
index 00000000..d488fa41
--- /dev/null
+++ b/modules/background_tasks/tests/test_background_tasks_settings.py
@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+import pytest
+from background_tasks.settings import BackgroundTasksSettings
+from pydantic import ValidationError
+
+
+@pytest.mark.parametrize("retention_days", [1, 3650])
+def test_retention_days_accepts_supported_boundaries(retention_days: int) -> None:
+    assert BackgroundTasksSettings(retention_days=retention_days).retention_days == retention_days
+
+
+@pytest.mark.parametrize("retention_days", [0, -5, 3651, 999_999_999])
+def test_retention_days_rejects_values_outside_supported_range(retention_days: int) -> None:
+    with pytest.raises(ValidationError):
+        BackgroundTasksSettings(retention_days=retention_days)
+
+
+@pytest.mark.parametrize("max_retries", [0, 100])
+def test_max_retries_accepts_supported_boundaries(max_retries: int) -> None:
+    assert BackgroundTasksSettings(max_retries=max_retries).max_retries == max_retries
+
+
+@pytest.mark.parametrize("max_retries", [-1, 101, 999_999_999])
+def test_max_retries_rejects_values_outside_supported_range(max_retries: int) -> None:
+    with pytest.raises(ValidationError):
+        BackgroundTasksSettings(max_retries=max_retries)
diff --git a/modules/settings/package.json b/modules/settings/package.json
index 52830f7a..3b8975a1 100644
--- a/modules/settings/package.json
+++ b/modules/settings/package.json
@@ -12,5 +12,7 @@
   "devDependencies": {
     "@simple-module-py/tsconfig": "*"
   },
-  "dependencies": {}
+  "dependencies": {
+    "sonner": "^2.0.7"
+  }
 }
diff --git a/modules/settings/settings/locales/en.json b/modules/settings/settings/locales/en.json
index 13562939..5f4499e1 100644
--- a/modules/settings/settings/locales/en.json
+++ b/modules/settings/settings/locales/en.json
@@ -88,6 +88,7 @@
   "modules_form": {
     "save": "Save",
     "saving": "Saving…",
+    "saved_toast": "Settings saved",
     "default_group": "General",
     "requires_restart": "Requires restart",
     "reset_to_default": "Reset to default",
diff --git a/modules/settings/settings/pages/components/ModuleForm.tsx b/modules/settings/settings/pages/components/ModuleForm.tsx
index 50122046..2d860c6f 100644
--- a/modules/settings/settings/pages/components/ModuleForm.tsx
+++ b/modules/settings/settings/pages/components/ModuleForm.tsx
@@ -36,23 +36,27 @@ export function ModuleForm({ module: m, testable = false }: Props) {
   }, [m.fields]);
 
   const [values, setValues] = useState<Record<string, unknown>>(initial);
+  const [baseline, setBaseline] = useState<Record<string, unknown>>(initial);
   const [errors, setErrors] = useState<Record<string, string>>({});
   const [busy, setBusy] = useState(false);
+  const [saved, setSaved] = useState(false);
 
   // Reset the edit buffer whenever the underlying module changes (package
   // switch, or server-reloaded props after a save/reset).
   useEffect(() => {
     setValues(initial);
+    setBaseline(initial);
     setErrors({});
+    setSaved(false);
   }, [initial]);
 
   const modifiedFields = useMemo(() => {
     const s = new Set<string>();
     for (const name of Object.keys(values)) {
-      if (notEqual(values[name], initial[name])) s.add(name);
+      if (notEqual(values[name], baseline[name])) s.add(name);
     }
     return s;
-  }, [values, initial]);
+  }, [values, baseline]);
 
   const defaultByName = useMemo(() => {
     const o: Record<string, unknown> = {};
@@ -74,6 +78,7 @@ export function ModuleForm({ module: m, testable = false }: Props) {
 
   async function onSave() {
     setBusy(true);
+    setSaved(false);
     setErrors({});
     const changed: Record<string, unknown> = {};
     for (const name of modifiedFields) changed[name] = values[name];
@@ -90,12 +95,16 @@ export function ModuleForm({ module: m, testable = false }: Props) {
       }
       setErrors(fieldErrs);
     } else if (resp.ok) {
-      router.reload({ only: ['modules'] });
+      // Keeping the form mounted lets the confirmation remain visible instead
+      // of being discarded by an immediate Inertia reload.
+      setBaseline({ ...values });
+      setSaved(true);
     }
     setBusy(false);
   }
 
   async function onReset(name: string) {
+    setSaved(false);
     await fetch(`/api/settings/modules/${m.package}/${name}`, { method: 'DELETE' });
     router.reload({ only: ['modules'] });
   }
@@ -107,7 +116,12 @@ export function ModuleForm({ module: m, testable = false }: Props) {
           <h2 className="text-xl font-semibold">{m.module_name}</h2>
           <p className="text-xs font-mono text-muted-foreground">{m.package}</p>
         </div>
-        <div className="flex items-start gap-2">
+        <div className="flex items-center gap-2">
+          {saved && (
+            <p role="status" className="text-sm font-medium text-emerald-700">
+              {t(keys.settings.modules_form.saved_toast)}
+            </p>
+          )}
           {testable && <TestConnectionButton pkg={m.package} />}
           <button
             type="button"
@@ -146,7 +160,10 @@ export function ModuleForm({ module: m, testable = false }: Props) {
                     id={`field-${m.package}-${f.name}`}
                     field={f}
                     value={values[f.name]}
-                    onChange={(name, v) => setValues((prev) => ({ ...prev, [name]: v }))}
+                    onChange={(name, v) => {
+                      setSaved(false);
+                      setValues((prev) => ({ ...prev, [name]: v }));
+                    }}
                   />
                   {isModified && (
                     <button
diff --git a/modules/settings/tests-js/ModuleForm.test.tsx b/modules/settings/tests-js/ModuleForm.test.tsx
new file mode 100644
index 00000000..4f687f95
--- /dev/null
+++ b/modules/settings/tests-js/ModuleForm.test.tsx
@@ -0,0 +1,63 @@
+import '@testing-library/jest-dom/vitest';
+import { configureI18n } from '@simple-module-py/i18n';
+import { fireEvent, render, screen, waitFor } from '@testing-library/react';
+import { afterEach, describe, expect, test, vi } from 'vitest';
+
+configureI18n({
+  locale: 'en',
+  messages: {
+    'settings.modules_form.saved_toast': 'Settings saved',
+  },
+});
+
+const mocks = vi.hoisted(() => ({ reload: vi.fn() }));
+
+vi.mock('@inertiajs/react', () => ({
+  router: { reload: mocks.reload },
+}));
+
+import { ModuleForm, type ModuleView } from '../settings/pages/components/ModuleForm';
+
+const moduleView: ModuleView = {
+  module_name: 'BackgroundTasks',
+  package: 'background_tasks',
+  env_prefix: 'SM_BG_TASKS_',
+  class_name: 'BackgroundTasksSettings',
+  fields: [
+    {
+      name: 'retention_days',
+      type: 'int',
+      value: 14,
+      default: 14,
+      description: '',
+      is_secret: false,
+      requires_restart: false,
+      group: null,
+      env_var: 'SM_BG_TASKS_RETENTION_DAYS',
+    },
+  ],
+};
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+  vi.clearAllMocks();
+});
+
+describe('ModuleForm', () => {
+  test('shows success feedback after settings are saved', async () => {
+    const fetchMock = vi.fn().mockResolvedValue({ ok: true, status: 200 });
+    vi.stubGlobal('fetch', fetchMock);
+    render(<ModuleForm module={moduleView} />);
+
+    fireEvent.change(screen.getByLabelText('retention_days'), { target: { value: '30' } });
+    fireEvent.click(screen.getByRole('button', { name: /modules_form\.save$/ }));
+
+    await waitFor(() => expect(screen.getByRole('status')).toBeVisible());
+    expect(fetchMock).toHaveBeenCalledWith(
+      '/api/settings/modules/background_tasks',
+      expect.objectContaining({ body: JSON.stringify({ retention_days: 30 }) }),
+    );
+    expect(mocks.reload).not.toHaveBeenCalled();
+    expect(screen.getByRole('status')).toHaveTextContent('Settings saved');
+  });
+});
diff --git a/modules/settings/tests/test_module_api.py b/modules/settings/tests/test_module_api.py
index 436e9adf..fd1b2bc9 100644
--- a/modules/settings/tests/test_module_api.py
+++ b/modules/settings/tests/test_module_api.py
@@ -32,6 +32,28 @@ async def test_put_validation_error_surfaces_422(authenticated_client, app):
     assert "i18n_default_locale" in resp.text
 
 
+@pytest.mark.asyncio
+async def test_put_rejects_and_does_not_persist_unbounded_background_task_values(
+    authenticated_client, app, db_session
+):
+    from settings.service import SettingService
+    from settings.store import SettingsStore
+
+    original = app.state.background_tasks.settings
+    resp = await authenticated_client.put(
+        "/api/settings/modules/background_tasks",
+        json={"retention_days": -5, "max_retries": 999_999_999},
+    )
+
+    assert resp.status_code == 422
+    assert {error["loc"][-1] for error in resp.json()["detail"]} == {
+        "retention_days",
+        "max_retries",
+    }
+    assert app.state.background_tasks.settings is original
+    assert await SettingsStore(SettingService(db_session)).get_overrides("background_tasks") == {}
+
+
 @pytest.mark.asyncio
 async def test_delete_field_resets_to_default(authenticated_client, app):
     await authenticated_client.put("/api/settings/modules/host", json={"multi_tenant": True})
diff --git a/modules/settings/tsconfig.json b/modules/settings/tsconfig.json
index f543be08..3da1f2d6 100644
--- a/modules/settings/tsconfig.json
+++ b/modules/settings/tsconfig.json
@@ -6,5 +6,5 @@
       "@simple-module-py/ui/*": ["../../packages/ui/src/*"]
     }
   },
-  "include": ["settings/**/*.ts", "settings/**/*.tsx"]
+  "include": ["settings/**/*.ts", "settings/**/*.tsx", "tests-js/**/*.ts", "tests-js/**/*.tsx"]
 }
diff --git a/modules/users/tests-js/invite-emails.test.tsx b/modules/users/tests-js/invite-emails.test.tsx
new file mode 100644
index 00000000..4254f71f
--- /dev/null
+++ b/modules/users/tests-js/invite-emails.test.tsx
@@ -0,0 +1,50 @@
+import '@testing-library/jest-dom/vitest';
+import { configureI18n } from '@simple-module-py/i18n';
+import { render, screen } from '@testing-library/react';
+import { describe, expect, test, vi } from 'vitest';
+
+configureI18n({
+  locale: 'en',
+  messages: {
+    'users.invite_fields.invalid_email_one': '{email} is not a valid email address.',
+    'users.invite_fields.invalid_email_other': '{count} addresses are not valid email addresses.',
+  },
+});
+
+import { InviteFields } from '../users/pages/Users/components/InviteFields';
+import { isPlausibleEmail, parseInviteEmails } from '../users/pages/Users/invite-emails';
+
+describe('invite email validation', () => {
+  test('parses pasted addresses using every supported separator', () => {
+    expect(parseInviteEmails('one@example.com; two@example.com\nthree@example.com')).toEqual([
+      'one@example.com',
+      'two@example.com',
+      'three@example.com',
+    ]);
+  });
+
+  test.each([
+    ['teammate@example.com', true],
+    ['not-an-email', false],
+    ['missing-domain@', false],
+    ['missing-tld@example', false],
+  ])('classifies %s', (email, expected) => {
+    expect(isPlausibleEmail(email)).toBe(expected);
+  });
+
+  test('warns about an invalid address before submit', () => {
+    render(
+      <InviteFields
+        emails="not-an-email"
+        onEmailsChange={vi.fn()}
+        count={0}
+        invalidEmails={['not-an-email']}
+        mailerDelivers
+      />,
+    );
+
+    expect(screen.getByRole('alert')).toHaveTextContent(
+      'not-an-email is not a valid email address.',
+    );
+  });
+});
diff --git a/modules/users/tsconfig.json b/modules/users/tsconfig.json
index 0b4e77c4..991c4134 100644
--- a/modules/users/tsconfig.json
+++ b/modules/users/tsconfig.json
@@ -6,5 +6,5 @@
       "@simple-module-py/ui/*": ["../../packages/ui/src/*"]
     }
   },
-  "include": ["users/**/*.ts", "users/**/*.tsx"]
+  "include": ["users/**/*.ts", "users/**/*.tsx", "tests-js/**/*.ts", "tests-js/**/*.tsx"]
 }
diff --git a/modules/users/users/locales/en.json b/modules/users/users/locales/en.json
index 15b59a38..e2b0920f 100644
--- a/modules/users/users/locales/en.json
+++ b/modules/users/users/locales/en.json
@@ -229,6 +229,8 @@
     "label": "Email addresses",
     "hint": "One per line, or separated by commas.",
     "recognised": "{count} recognised.",
+    "invalid_email_one": "{email} is not a valid email address.",
+    "invalid_email_other": "{count} addresses are not valid email addresses.",
     "no_mailer": "This deployment logs invite mail instead of sending it. You will get a copyable link for each address to pass on yourself."
   },
   "create_fields": {
diff --git a/modules/users/users/pages/Users/AddPeople.tsx b/modules/users/users/pages/Users/AddPeople.tsx
index f4404258..25e5e7d8 100644
--- a/modules/users/users/pages/Users/AddPeople.tsx
+++ b/modules/users/users/pages/Users/AddPeople.tsx
@@ -11,6 +11,7 @@ import { CreateUserFields } from './components/CreateUserFields';
 import { InviteFields } from './components/InviteFields';
 import { type InviteResult, InviteResults } from './components/InviteResults';
 import { type Role, RolePicker } from './components/RolePicker';
+import { isPlausibleEmail, parseInviteEmails } from './invite-emails';
 
 type Mode = 'invite' | 'create';
 
@@ -59,11 +60,9 @@ function AddPeople() {
       prev.includes(roleName) ? prev.filter((r) => r !== roleName) : [...prev, roleName],
     );
 
-  /** Split a pasted block on commas, semicolons, and any whitespace. */
-  const parsedEmails = emails
-    .split(/[\s,;]+/)
-    .map((value) => value.trim())
-    .filter(Boolean);
+  const parsedEmails = parseInviteEmails(emails);
+  const invalidEmails = parsedEmails.filter((value) => !isPlausibleEmail(value));
+  const validEmailCount = parsedEmails.length - invalidEmails.length;
 
   async function submitInvite() {
     const resp = await fetch('/api/users/admin/invite/bulk', {
@@ -134,7 +133,9 @@ function AddPeople() {
   }
 
   const canSubmit =
-    mode === 'invite' ? parsedEmails.length > 0 : email.trim() !== '' && password !== '';
+    mode === 'invite'
+      ? validEmailCount > 0 && invalidEmails.length === 0
+      : email.trim() !== '' && password !== '';
 
   const submitLabel = loading
     ? t(keys.users.add_people.submitting)
@@ -187,7 +188,8 @@ function AddPeople() {
               <InviteFields
                 emails={emails}
                 onEmailsChange={setEmails}
-                count={parsedEmails.length}
+                count={validEmailCount}
+                invalidEmails={invalidEmails}
                 mailerDelivers={mailerDelivers}
               />
             ) : (
diff --git a/modules/users/users/pages/Users/components/InviteFields.tsx b/modules/users/users/pages/Users/components/InviteFields.tsx
index 7ce4fea5..18e6db6a 100644
--- a/modules/users/users/pages/Users/components/InviteFields.tsx
+++ b/modules/users/users/pages/Users/components/InviteFields.tsx
@@ -6,12 +6,19 @@ import { Info } from 'lucide-react';
 interface Props {
   emails: string;
   onEmailsChange: (value: string) => void;
-  /** Addresses parsed out of the box so far. */
+  /** Valid addresses parsed out of the box so far. */
   count: number;
+  invalidEmails: string[];
   mailerDelivers: boolean;
 }
 
-export function InviteFields({ emails, onEmailsChange, count, mailerDelivers }: Props) {
+export function InviteFields({
+  emails,
+  onEmailsChange,
+  count,
+  invalidEmails,
+  mailerDelivers,
+}: Props) {
   const { t } = useT();
   return (
     <div className="space-y-2">
@@ -36,6 +43,15 @@ export function InviteFields({ emails, onEmailsChange, count, mailerDelivers }:
         {count > 0 && t(keys.users.invite_fields.recognised, { count })}
       </p>
 
+      {invalidEmails.length > 0 && (
+        <p role="alert" className="text-xs text-destructive">
+          {t(keys.users.invite_fields.invalid_email, {
+            count: invalidEmails.length,
+            email: invalidEmails[0],
+          })}
+        </p>
+      )}
+
       {!mailerDelivers && (
         <p className="flex items-start gap-1.5 rounded-md bg-amber-50 p-2 text-xs text-amber-900">
           <Info className="mt-0.5 size-3.5 shrink-0" />
diff --git a/modules/users/users/pages/Users/invite-emails.ts b/modules/users/users/pages/Users/invite-emails.ts
new file mode 100644
index 00000000..fb6649b0
--- /dev/null
+++ b/modules/users/users/pages/Users/invite-emails.ts
@@ -0,0 +1,17 @@
+const SIMPLE_EMAIL_PATTERN = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
+
+/** Split a pasted block on commas, semicolons, and any whitespace. */
+export function parseInviteEmails(value: string): string[] {
+  return value
+    .split(/[\s,;]+/)
+    .map((email) => email.trim())
+    .filter(Boolean);
+}
+
+/**
+ * Catch obvious typos before submit while leaving authoritative validation to
+ * the backend's EmailStr validator.
+ */
+export function isPlausibleEmail(value: string): boolean {
+  return SIMPLE_EMAIL_PATTERN.test(value);
+}
diff --git a/package-lock.json b/package-lock.json
index a6238b20..947d0ffb 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -197,6 +197,9 @@
     "modules/settings": {
       "name": "@simple-module-py/settings",
       "version": "0.1.0",
+      "dependencies": {
+        "sonner": "^2.0.7"
+      },
       "devDependencies": {
         "@simple-module-py/tsconfig": "*"
       },
diff --git a/packages/i18n/src/generated-resources.ts b/packages/i18n/src/generated-resources.ts
index 478fe935..fcaafd21 100644
--- a/packages/i18n/src/generated-resources.ts
+++ b/packages/i18n/src/generated-resources.ts
@@ -506,6 +506,7 @@ export default {
     'settings.modules_form.requires_restart': '',
     'settings.modules_form.reset_to_default': '',
     'settings.modules_form.save': '',
+    'settings.modules_form.saved_toast': '',
     'settings.modules_form.saving': '',
     'settings.modules_form.set_new_value': '',
     'settings.nav.settings': '',
@@ -698,6 +699,8 @@ export default {
     'users.index.tab_users': '',
     'users.index.title': '',
     'users.invite_fields.hint': '',
+    'users.invite_fields.invalid_email_one': '',
+    'users.invite_fields.invalid_email_other': '',
     'users.invite_fields.label': '',
     'users.invite_fields.no_mailer': '',
     'users.invite_fields.recognised': '',
diff --git a/packages/i18n/src/keys.generated.ts b/packages/i18n/src/keys.generated.ts
index 1043e40b..a81e8803 100644
--- a/packages/i18n/src/keys.generated.ts
+++ b/packages/i18n/src/keys.generated.ts
@@ -664,6 +664,7 @@ export const keys = {
       requires_restart: 'settings.modules_form.requires_restart',
       reset_to_default: 'settings.modules_form.reset_to_default',
       save: 'settings.modules_form.save',
+      saved_toast: 'settings.modules_form.saved_toast',
       saving: 'settings.modules_form.saving',
       set_new_value: 'settings.modules_form.set_new_value',
     },
@@ -914,6 +915,9 @@ export const keys = {
     },
     invite_fields: {
       hint: 'users.invite_fields.hint',
+      invalid_email: 'users.invite_fields.invalid_email',
+      invalid_email_one: 'users.invite_fields.invalid_email_one',
+      invalid_email_other: 'users.invite_fields.invalid_email_other',
       label: 'users.invite_fields.label',
       no_mailer: 'users.invite_fields.no_mailer',
       recognised: 'users.invite_fields.recognised',
diff --git a/vitest.config.mts b/vitest.config.mts
index e4a207aa..20d345f0 100644
--- a/vitest.config.mts
+++ b/vitest.config.mts
@@ -15,6 +15,8 @@ export default defineConfig({
       'packages/**/*.test.tsx',
       'host/client_app/**/*.test.ts',
       'host/client_app/**/*.test.tsx',
+      'modules/**/tests-js/**/*.test.ts',
+      'modules/**/tests-js/**/*.test.tsx',
       // The lint gates are code too — a "fix" to one of their heuristics
       // should not be able to quietly stop them detecting anything.
       'scripts/**/*.test.mts',