feat(plex): Phase B — Siftlode→Plex watch-state push

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).

backend/app/plex/client.py

@@ -166,6 +166,38 @@ class PlexClient:
         items = mc.get("Metadata", []) or []
         return items[0] if items else None
 
+    # --- Watch-state write-back (P5 Phase B — Siftlode→Plex push; acts on the token's account) ---
+    _LIBRARY_IDENTIFIER = "com.plexapp.plugins.library"
+
+    def scrobble(self, rating_key: str) -> None:
+        """Mark the item *watched* on the token's Plex account (viewCount++, resume cleared)."""
+        self._write(
+            "GET",
+            f"/:/scrobble?identifier={self._LIBRARY_IDENTIFIER}"
+            f"&key={quote(str(rating_key), safe='')}",
+        )
+
+    def unscrobble(self, rating_key: str) -> None:
+        """Mark the item *unwatched* on the token's Plex account (clears viewCount)."""
+        self._write(
+            "GET",
+            f"/:/unscrobble?identifier={self._LIBRARY_IDENTIFIER}"
+            f"&key={quote(str(rating_key), safe='')}",
+        )
+
+    def set_timeline(
+        self, rating_key: str, time_ms: int, duration_ms: int, state: str = "stopped"
+    ) -> None:
+        """Report a resume position (viewOffset) for the item on the token's account. `state` is a
+        Plex timeline state (playing|paused|stopped); we push the settled position as ``stopped``."""
+        key = quote(f"/library/metadata/{rating_key}", safe="")
+        self._write(
+            "GET",
+            f"/:/timeline?identifier={self._LIBRARY_IDENTIFIER}"
+            f"&ratingKey={quote(str(rating_key), safe='')}&key={key}"
+            f"&state={state}&time={int(time_ms)}&duration={int(duration_ms)}",
+        )
+
     def raw_get(self, path: str) -> bytes:
         """Raw bytes of an arbitrary Plex resource path (e.g. an external subtitle stream `key` like
         ``/library/streams/553184``). Keeps the admin token server-side."""