Make Plex the source of truth for watch state:
- Write-back during playback: item_progress now pushes the resume position to
Plex on the periodic checkpoint too (not only the final flush), throttled to
one push per item every ~10s (in-memory, single-process rate limiter). The FE
now also fires an immediate (debounced) push on seek.
- Faster incremental pull: 30 → 15 min (plex_watch_sync_interval_min).
- Plex-authoritative pull: _pull_apply resolves a genuine divergence to Plex's
state instead of last-write-wins; the _same_state + synced_to_plex short-circuit
still prevents a freshly-pushed row from bouncing. Removed the now-unused
_LWW_TOLERANCE_S / _sift_ts.
Tests: 5 new in test_plex_watch_sync.py (Plex-wins resume/watched, no-bounce,
throttle final vs periodic).
