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).

frontend/src/lib/api.ts

@@ -1244,16 +1244,29 @@ export const api = {
   // cue times onto the HLS session's zero-based clock so subtitles line up with speech after a seek.
   plexSubtitleUrl: (id: string, ord: number, offset = 0): string =>
     `/api/plex/subtitle/${encodeURIComponent(id)}/${ord}${offset ? `?offset=${offset}` : ""}`,
-  plexProgress: (id: string, position_seconds: number, duration_seconds: number): Promise<unknown> =>
+  // `final` marks a settled checkpoint (pause / page-hide / leaving the player) as opposed to the
+  // periodic 10s tick — the backend mirrors the resume position to a linked Plex account only on a
+  // final checkpoint, so a single watch doesn't spray Plex with timeline updates.
+  plexProgress: (
+    id: string,
+    position_seconds: number,
+    duration_seconds: number,
+    final = false,
+  ): Promise<unknown> =>
     req(`/api/plex/item/${encodeURIComponent(id)}/progress`, {
       method: "POST",
-      body: JSON.stringify({ position_seconds, duration_seconds }),
+      body: JSON.stringify({ position_seconds, duration_seconds, final }),
     }),
   // Fire-and-forget progress save that SURVIVES page unload (F5/close/navigate): a normal fetch is
   // cancelled on unload and React effect cleanup doesn't run on a full reload, so the last position
   // (esp. right after a seek) would be lost. `keepalive` lets the POST complete during unload; we
   // replicate req()'s credentials + account header (no retry — there's no page left to retry on).
-  plexProgressBeacon: (id: string, position_seconds: number, duration_seconds: number): void => {
+  plexProgressBeacon: (
+    id: string,
+    position_seconds: number,
+    duration_seconds: number,
+    final = false,
+  ): void => {
     const active = getActiveAccount();
     try {
       void fetch(`/api/plex/item/${encodeURIComponent(id)}/progress`, {
@@ -1264,7 +1277,7 @@ export const api = {
           "Content-Type": "application/json",
           ...(active != null ? { [ACTIVE_ACCOUNT_HEADER]: String(active) } : {}),
         },
-        body: JSON.stringify({ position_seconds, duration_seconds }),
+        body: JSON.stringify({ position_seconds, duration_seconds, final }),
       }).catch(() => {});
     } catch {
       /* ignore */