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.
