Mirror watched/unwatched/resume from Siftlode to a linked (owner) Plex account.
- PlexClient: scrobble / unscrobble / set_timeline (/:/scrobble, /:/unscrobble, /:/timeline).
- watch_sync: link_for_push() gates on owner + sync_enabled + initial_import_done; best-effort
push_state_to_plex() runs as a background task with its own session, never raises (Plex being
down must not break the user's action), and flips synced_to_plex on success so Phase C's pull
won't bounce it back.
- item_state / item_progress schedule the push: watched/unwatched immediately, resume only on a
"final" checkpoint (pause / pagehide / unmount) — not every 10s tick — so one watch doesn't spray
/:/timeline at the server. `hidden` is Siftlode-only and never touches Plex.
- Frontend: plexProgress / plexProgressBeacon carry a `final` flag; the periodic checkpoint is
non-final, pause/pagehide/leaving the player are final.
Verified live against the real Plex server (scrobble/unscrobble/timeline round-trip + restore;
link gating; synced_to_plex flip).
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