refactor(plex): address /code-review on the R12 S1 split

Cleanup only, no behaviour change (ruff clean + pytest 193):
- imports hoisted above `router =` and grouped/sorted (isort) across the split
  files; ruff-format pass restores the 2-blank-line PEP8 spacing the generator
  collapsed to 1.
- the shared on-disk cache root moves to a neutral app/plex/cache.py, so
  subtitles.py no longer imports it from imgcache.py (drops the cross-import).
- the former plex.py module docstring's access-model note (read endpoints open
  to any authed user; admin endpoints test/sync) is restored in the package
  __init__.py.

backend/app/routes/plex/__init__.py

@@ -1,8 +1,13 @@
 """Plex route package — assembles the admin/browse/playback sub-routers.
 
+Read endpoints (libraries/browse/show/image) are available to ANY authenticated user (demo +
+pure-Siftlode included) — this is an access layer to the Plex library WITHOUT requiring a plex.tv
+account, and playback is a local file. Admin endpoints test the connection and run the catalog sync.
+
 Split out of the former single routes/plex.py (R12 S1). `router` keeps the same shape the app
 wiring expects (`plex_routes.router`), so main.py is unchanged.
 """
+
 from fastapi import APIRouter
 
 from app.routes.plex import admin, browse, playback