examples/app-showcase: add an end-to-end external datasource + federated object example

[xuyushun441-sys]

Why

The runtime datasource experience now ships end-to-end:

• Studio manages datasources through the generic metadata-admin engine, reachable from the Setup app's left menu (Setup → Integrations → Datasources) — feat(studio): manage datasource through the metadata-admin engine (drop bespoke page) objectui#1846, framework feat(datasource): Setup-app "Datasources" nav entry → metadata-admin engine #2091.
• Read-only introspection + sync routes: GET /datasources/drivers (feat(datasource): expose driver catalog with config JSON Schema #2083), GET /datasources/:name/remote-tables, POST /datasources/:name/object-draft, POST /datasources/:name/test, GET /datasources/:name (feat(datasource): read-only introspection routes + by-name test for Studio sync #2085, feat(datasource): GET /datasources/:name detail for Studio edit form #2086).
• Runtime datasources now persist across restart (fix(datasource): persist runtime datasources across restart #2096).
• Generic secret + dynamic-config SchemaForm widgets (objectui#1853).

But examples/app-showcase has no external-datasource example — its only datasource is the managed in-memory sqlite. So there is nothing in the canonical example app that demonstrates (or regression-guards) the federation + datasource-admin flow. New users can't see "connect an external DB → introspect → query its tables as objects", and contributors have no fixture to dogfm against.

This example is considered high-value: the datasource story is a flagship capability and the showcase is where people learn the platform.

Goal

A runnable, self-contained example in app-showcase that demonstrates the full external-datasource path without requiring an external server (so os dev just works):

1. A code-defined external datasource (*.datasource.ts, schemaMode: 'external') pointing at a second, pre-seeded sqlite file (or equivalent) so there are real remote tables to introspect.
2. One or more external/federated objects bound to that datasource's remote tables (external: { remoteName, remoteSchema? }), queryable through the normal ObjectQL/REST surface.
3. Wiring in objectstack.config.ts (+ a datasourceMapping rule if needed) and a short README/comment explaining the flow.

End state: boot showcase → the external datasource shows in Setup → Datasources; its objects appear and are queryable; and an admin can also reproduce the runtime flow (Setup → Datasources → Sync objects) against it.

Proposed shape (minimal, no external server)

• examples/app-showcase/src/datasources/<name>.datasource.ts — sqlite, schemaMode: 'external', filename = a seeded fixture db (e.g. created at boot / committed fixture).
• examples/app-showcase/src/objects/<remote>.object.ts — external object(s) binding the fixture's tables (e.g. customers, orders).
• objectstack.config.ts — import the datasource + objects (+ mapping).
• Decide how the fixture tables get created (open question below).

Open questions / decisions for the implementer

• Fixture provisioning: how does the "external" sqlite get its tables at dev time? Options: a committed .db fixture; a tiny boot/seed step that creates+populates a side sqlite; or a memory driver pre-seeded via a hook. Pick the one that keeps os dev zero-config.
• Showcase vs a dedicated example app: add to app-showcase (conditionally imported so the default stays in-memory) vs. a new examples/app-showcase-external/. Lean toward in-showcase + clearly isolated, unless it muddies the default.
• Scope: code-defined only, or also document the runtime/UI sync flow (the wizard already exists)? At minimum the code-defined example; optionally a README note pointing at the Setup → Datasources sync UI.
• Secret demo: optionally include a driver that exercises a secret credential field (e.g. a commented postgres variant) to showcase the masked secret widget — without making the default example require a real server.

Acceptance criteria

• os dev on app-showcase boots with no extra setup; the external datasource + its federated objects are present.
• The external datasource appears in Setup → Datasources and GET /api/v1/datasources / GET /api/v1/meta/datasource.
• The federated objects are listable/queryable via REST.
• Short docs/comment explaining the example and pointing at the sync UI.
• (Nice) a smoke test asserting the external objects resolve.

References

• ADR-0015 (external datasource federation) + Addendum (runtime admin).
• packages/services/service-datasource/README.md (Federation + Runtime admin halves).
• PRs: framework feat(datasource): expose driver catalog with config JSON Schema #2083, feat(datasource): read-only introspection routes + by-name test for Studio sync #2085, feat(datasource): GET /datasources/:name detail for Studio edit form #2086, feat(datasource): Setup-app "Datasources" nav entry → metadata-admin engine #2091, fix(datasource): persist runtime datasources across restart #2096; objectui fix(i18n): correct stale page-type help text in metadata-form bundles #1846, chore: version packages #1853.