feat(plex): Phase C — incremental + full Plex↔Siftlode watch reconcile

Completes the two-way watch-state sync with two scheduler jobs:

- plex_watch_sync (default 30m): pull recent Plex-side changes (watch history + on-deck, filtered
  to the owner account) into Siftlode under last-write-wins (_pull_apply + a _same_state ping-pong
  guard + skew tolerance), then re-push any still-unsynced local states.
- plex_watch_reconcile (default daily): full section rescan; uses synced_to_plex to settle what the
  incremental feed can't — notably propagating a Plex-side un-watch (clear a previously-mirrored row
  Plex no longer has) — while re-pushing never-synced local states and never touching hidden
  (Siftlode-only) rows. Union-preserving.

PlexClient gains accounts/watch_history/on_deck; _scan_plex_states is factored out and shared with
the one-time import. Owner accountID is resolved once and cached on the link. Both jobs are
registered in the scheduler (pause-skip, activity tracking, run-now, admin-tunable intervals) with
trilingual (HU/EN/DE) labels + descriptions. New config default plex_watch_reconcile_interval_min.

Verified live against the real Plex server: read feeds, last-write-wins, dirty re-push, the
incremental job end-to-end, and a full reconcile that cleared exactly the one un-watched item with
zero collateral across 17976 scanned.

backend/app/config.py

@@ -202,8 +202,13 @@ class Settings(BaseSettings):
     plex_client_id: str = "siftlode-server"
     # How often the catalog sync job mirrors Plex metadata, in minutes.
     plex_sync_interval_min: int = 360
-    # How often the incremental Plex→Siftlode watch-state reconcile runs, in minutes (Phase C).
+    # How often the incremental Plex→Siftlode watch-state reconcile runs, in minutes (Phase C):
+    # a cheap history + on-deck pull plus a re-push of any states that failed their immediate push.
     plex_watch_sync_interval_min: int = 30
+    # How often the FULL watch-state reconcile runs, in minutes (Phase C). A whole-section rescan
+    # that catches what the incremental history feed can't — notably an un-watch done on the Plex
+    # side — so it runs far less often (daily) than the incremental pass.
+    plex_watch_reconcile_interval_min: int = 1440
     # Max concurrent transcodes for the P3 fallback. Low by default — CPU-only transcode is
     # expensive; direct-serve (browser-compatible files) has no such limit.
     plex_max_transcodes: int = 1