Pin a stable Plex client identity to stop New Device notifications (#190)

[Brandon-Haney]

Summary

Fixes #190. Plex was firing "New Device" security notifications attributed to PlexCache, often in the middle of the night, because PlexCache connected with the plexapi default X-Plex-Client-Identifier. That default is derived from the container's MAC/hostname (hex(uuid.getnode())), so Plex sees an unrecognized device whenever the container is recreated or after Plex ages the headless client out, and re-registers it.

This pins a stable identity for every Plex connection by reusing the plexcache_client_id that's already persisted for the OAuth login. The device now also reports as the PlexCache-D product/device name at the app version, instead of "PlexAPI" at the plexapi library version.

What changed

• core/plex_api.py — apply_plex_client_identity() sets the plexapi header globals and rebuilds BASE_HEADERS in place (so modules that imported it by name pick up the change). ensure_plex_client_identity() loads, or creates and persists, the id from settings and applies it. It leaves a missing (pre-setup) or unparseable settings file untouched rather than writing a stub.
• core/app.py — applies the identity right after config load, before any Plex connection, and keeps the in-memory config in sync so a later settings save can't drop the id.
• web/main.py — applies the identity in the web lifespan before the startup Plex prefetch.
• tests/test_plex_client_identity.py — covers header mutation, in-place BASE_HEADERS update, reuse of an existing id, create-and-persist, and the missing/unparseable-file guards.

Installs configured by OAuth already have plexcache_client_id; installs configured by manual token get one minted and saved once on first run.

How to test

1. In Plex, make sure new-device notifications are enabled, and confirm PlexCache currently shows up in your authorized devices with a random hex id (e.g. 68fdc57b2575) and the PlexAPI product.
2. Run PlexCache (CLI or Web). With --verbose you'll see a debug line: Plex client identity set: id=… product=PlexCache-D version=….
3. In Plex authorized devices, confirm a single device now shows the persisted id with product/name PlexCache-D at the PlexCache version.
4. Recreate the container and run again. The device id stays the same and no new-device notification fires. Before this change, a recreate produced a fresh id and another notification.

Confirmed working in the field by the reporter on #190 (no further notifications after switching to the build with this fix).