refactor(subsonic): split routes/subsonic.ts into endpoint modules with a query layer (#243)

[benders]

Closes #243.

Pure refactor, zero behavior change: the 1,819-line hub/src/routes/subsonic.ts monolith is now endpoint-family modules with a prepared-statement query layer. Four commits, one per phase:

Commit	Phase
834bc46	1 — builders: buildSong, buildAlbum, annotateStarred, annotatePlays, contentTypeForFormat, audioSourceFields, share-ID pickers + row types → routes/subsonic/{builders,types}.ts
b4d1a11	2 — query layer: 26 static prepared statements → createSubsonicQueries() in hub/src/db/queries/subsonic-queries.ts, all prepared eagerly at plugin init (extends the #130 hoisting convention). Explicit SubsonicQueries interface so statement-name typos fail typecheck. New init-time smoke test catches SQL/schema drift at boot.
9570b23	3 — module split: monolith deleted; handlers moved verbatim into index.ts (99), browsing.ts (573), search.ts (84), playlists.ts (34), annotations.ts (210), stream.ts (358), composed via a shared SubsonicRouteContext. Only external import change is server.ts.
e8672cd	4 — docs sweep: 5 docs updated to point at the new module paths.

Deliberately unchanged

• Endpoint registration set, preHandlers, and the castToken path-gating (requireSubsonicAuthBinary) — no auth files touched.
• getAlbumList2's dynamically-assembled SQL stays inline in browsing.ts (it can't be eagerly prepared).
• XML-default response format (Subsonic responses default to JSON; spec says XML #164) and all response shapes.
• BEST_SOURCE_SUBQUERY re-exported byte-for-byte even though it appears to be dead code (flagged on the issue for a follow-up — deleting dead code is out of scope for a pure refactor).

Verification

• pnpm verify green: 55 hub + 16 frontend test files (676+ tests), typecheck clean, lint:boundary zero output.
• pnpm lint zero findings in both hub/ and frontend/ (the root-level combined runner OOMs locally — pre-existing env quirk, unrelated; CI lints per-package).
• Federation compat suite green: 84/84 passed against the Docker hub-a/b/c stack — the strongest proof of zero Subsonic behavior change.
• All Subsonic behavior suites (subsonic-routes, subsonic, stream, star-routes, play-events, subsonic-internal-auth, cast-token-stream.integration) pass unmodified.

Notes

• Acceptance criteria from refactor(subsonic): split routes/subsonic.ts into endpoint modules with a query layer #243 met: monolith deleted; largest module is browsing.ts at 573 lines (23 over the ~550 allowance noted in the issue, because getAlbumList2's dynamic SQL stays inline); no db.prepare inside request handlers in the new modules.
• Registration order: every route is a distinct static path with its own preHandler, so the slight interleaving of registration calls (documented in index.ts) has no effect on Fastify routing.

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