feat(plex): P0 backend foundations — catalog model, config, client
Optional Plex module groundwork (design locked 2026-07-05): plays the LOCAL
physical file; Plex is metadata + optional watch-sync only.
- models + migration 0044: plex_libraries/plex_shows/plex_seasons/plex_items
(playable leaf, ffprobe playability class, markers, weighted FTS search_vector)
+ plex_states (per-user watch state, mirrors video_states)
- sysconfig 'plex' group + config.py env defaults (server url/token[secret]/
path-map/libraries/sync-interval/max-transcodes)
- app/plex/client.py (PlexClient httpx: server info, sections, items, metadata
+markers, image) + app/plex/paths.py (Plex→local path map + file resolve)
- routes/plex.py admin POST /api/plex/test (verify connection + list sections)
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