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.

frontend/src/i18n/locales/en/scheduler.json

@@ -69,7 +69,9 @@
     "maintenance": "Finds videos that can no longer be played (deleted, made private, abandoned premieres), hides them from the feed and removes them after a grace period; re-checks the least-recently-checked videos. If it stops, dead videos linger in the catalogue.",
     "explore_cleanup": "Removes channels you explored but never subscribed to (and their videos) after a grace period, so curiosity browsing doesn't pile up in the catalogue. If it stops, abandoned explored channels linger.",
     "download_gc": "Deletes downloaded files past their retention window, reclaims space no one is holding anymore, and warns owners before a shared file expires. If it stops, old downloads pile up and disk space isn't freed.",
-    "plex_sync": "Mirrors the enabled Plex libraries into the app's catalogue — the metadata behind Plex browsing, search, filters and the info pages (genres, cast, ratings, artwork). If it stops, newly added or changed Plex titles don't appear until the next sync."
+    "plex_sync": "Mirrors the enabled Plex libraries into the app's catalogue — the metadata behind Plex browsing, search, filters and the info pages (genres, cast, ratings, artwork). If it stops, newly added or changed Plex titles don't appear until the next sync.",
+    "plex_watch_sync": "Pulls recent Plex watch changes (finished episodes, resume positions) into Siftlode and re-pushes any local watch changes that didn't reach Plex. The cheap two-way keep-in-sync between full reconciles. If it stops, watched/resume made in the Plex app takes longer to show here.",
+    "plex_watch_reconcile": "A full watch-state reconcile that rescans every library — catches what the incremental pass can't, notably an episode un-marked as watched on the Plex side. Heavier, so it runs about once a day. If it stops, un-watches done in Plex aren't mirrored back."
   },
   "jobs": {
     "rss_poll": "RSS poll (new uploads)",
@@ -83,7 +85,9 @@
     "demo_reset": "Demo account reset",
     "explore_cleanup": "Explored-channel cleanup",
     "download_gc": "Download cleanup",
-    "plex_sync": "Plex library sync"
+    "plex_sync": "Plex library sync",
+    "plex_watch_sync": "Plex watch sync (incremental)",
+    "plex_watch_reconcile": "Plex watch reconcile (full)"
   },
   "queue": {
     "recentPending": "Channels to sync",