refactor(plex): split browse.py god-module into per-concern routers (E3 S1)

routes/plex/browse.py was a 1693-line god-module (libraries + facets +
collections + playlists + show/item detail + people). Split it into focused
sibling routers under the same /api/plex prefix — pure code motion, no SQL /
route-path / response-shape changes:

  browse.py  1693 -> 469  (list_libraries, unified_library, facets)
  collections.py  (new 253)  browse + admin collection editor; owns _collection_source
  playlists.py    (new 405)  the user's orderable playlists
  detail.py       (new 438)  show/item detail + enrichment; _lib_key lives here
  people.py       (new 139)  cast/crew chips + person filmography

Cross-module helpers _norm_lang + _content_libs move to _shared.py (their one
home); detail imports _collection_source from collections (one-way, no cycle).
__init__.py includes the four new routers. Three DB-lane tests follow the moved
symbols (import path for _content_libs; monkeypatch targets for PlexClient /
_show_enrichment now on detail).

Behaviour-equivalent: ruff clean, 348 DB-lane pass (unchanged), parity net green.

backend/tests/test_plex_overlay.py

@@ -11,7 +11,7 @@ import app.plex.sync as syncmod
 from app import sysconfig
 from app.models import PlexItem, PlexLibrary
 from app.plex.sync import _apply_item, _canonical_guid, _sync_overlay
-from app.routes.plex.browse import _content_libs
+from app.routes.plex._shared import _content_libs
 
 
 class _StubPlex: