Extract DB module to phoenix_kit_db package; remove dead PagesHTML

[mdon]

Summary

Two cleanups in core, both part of the lib/modules/* extraction direction:

1. DB module extraction — `lib/modules/db/` is now a standalone PhoenixKit plugin package (`phoenix_kit_db`, separate repo). Hosts pull it in via `{:phoenix_kit_db, "~> 0.1"}`; auto-discovery + `admin_tabs/0` wires up the routes — no hand-registration in core needed.
2. Dead PagesHTML removal — `PhoenixKitWeb.PagesHTML` had no callers, no controller, no route. The `embed_templates` directive plus `pages_html/show.html.heex` plus the `integration.ex` docstring described a markdown-page-rendering feature that was never wired up. Publishing module covers actual CMS-page rendering.

The companion DB module repo is in flight on the boss's side; this PR is the core-side cleanup and can ship independently — the DB explorer route registration is now done by the external module via `admin_tabs/0` once the package is installed.

Commits

• `6b5badb4` — Remove DB module — extracted to phoenix_kit_db package (the surgical lift: drop `PhoenixKit.Modules.DB` from `internal_modules/0`, drop the three hand-registered `live` declarations, drop the hardcoded DB module card from `modules.html.heex`, delete `lib/modules/db/` entirely — 8 files).
• `3a859a25` — Update internal-modules tests after DB extraction (drop `PhoenixKit.Modules.DB` from `@all_internal_modules` in `module_test.exs` and the `expected` list in `module_registry_test.exs`; lower three `>= 8` count assertions to `>= 7`).
• `c42fb797` — Update module-system guide reference table after DB extraction (move `lib/modules/db/db.ex` reference from the Internal examples section to External as `phoenix_kit_db/` between hello_world and document_creator — sits naturally between them as a small-footprint plugin with a Postgrex.Notifications GenServer child + multi-page admin tabs).
• `559f86df` — Remove dead PagesHTML module (delete `pages_html.ex` + `pages_html/show.html.heex`; trim the "Public pages routes" docstring block in `integration.ex`).

Verification

• `mix compile --warnings-as-errors`: clean.
• `mix format --check-formatted`: clean.
• `mix credo --strict`: 0 issues across 514 files; module count drops 7204 → 7202 (one removed for DB, one for PagesHTML).
• `mix test`: 1055 tests, 4 failures — all in `V107Test` (AI endpoint `integration_uuid` backfill); pre-existing on `dev` before this branch (verified by stashing my changes and re-running). Unrelated to this PR.

Stale-ref grep — both removals

```
ast-grep --lang elixir --pattern 'PhoenixKit.Modules.DB' lib/ test/ # zero
ast-grep --lang elixir --pattern 'PhoenixKitWeb.PagesHTML' lib/ test/ # zero
rg -n 'db_explorer|/admin/db' lib/ # zero (besides the
# auth.ex:997 best-path
# map entry, kept by
# design — same shape
# as the also-external
# phoenix_kit_ai entry)
rg -n 'PagesHTML|PagesController|pages_html' lib/ test/ dev_docs/ guides/ # zero
```

The four hits in `dev_docs/{guides,plans,audits}/` for `lib/modules/db` are point-in-time PR reviews / plans / audits — historical artifacts, left untouched per the workspace reviewer-artifact convention. The single living guide reference (`module-system-guide.md`) was updated in commit `c42fb797`.

Test plan

• `mix compile --warnings-as-errors` clean
• `mix format --check-formatted` clean
• `mix credo --strict` zero issues
• `mix test`: 1055 tests, only the 4 pre-existing V107 failures (unrelated to this PR)
• 3x `mix test` stability runs — same 4 V107 failures on each, no flakes introduced
• `ast-grep` confirms zero structural references to either removed module