feat(plex): Plex-authoritative watch-sync — resume write-back + faster pull (S2)

Make Plex the source of truth for watch state:

- Write-back during playback: item_progress now pushes the resume position to
  Plex on the periodic checkpoint too (not only the final flush), throttled to
  one push per item every ~10s (in-memory, single-process rate limiter). The FE
  now also fires an immediate (debounced) push on seek.
- Faster incremental pull: 30 → 15 min (plex_watch_sync_interval_min).
- Plex-authoritative pull: _pull_apply resolves a genuine divergence to Plex's
  state instead of last-write-wins; the _same_state + synced_to_plex short-circuit
  still prevents a freshly-pushed row from bouncing. Removed the now-unused
  _LWW_TOLERANCE_S / _sift_ts.

Tests: 5 new in test_plex_watch_sync.py (Plex-wins resume/watched, no-bounce,
throttle final vs periodic).

backend/app/plex/watch_sync.py

@@ -11,11 +11,14 @@ already pages) and upsert them into the owner's `plex_states`. Plex wins on over
 exist only in Siftlode are left untouched (union, Plex-authoritative on the intersection).
 
 Phases B (Siftlode→Plex push via scrobble/timeline) and C (incremental Plex→Siftlode via the watch
-history + onDeck, last-write-wins) build on this and land in later ships.
+history + onDeck) build on this and land in later ships. As of S2 Plex is the source of truth: the
+player pushes the resume position back to Plex on the periodic checkpoint (not just the final flush),
+and the incremental pull resolves a genuine divergence to Plex's state (see `_pull_apply`) rather than
+running a last-write-wins comparison.
 """
 
 import logging
-from datetime import datetime, timedelta, timezone
+from datetime import datetime, timezone
 
 from sqlalchemy import func, select, update
 from sqlalchemy.dialects.postgresql import insert as pg_insert
@@ -293,12 +296,6 @@ def push_bulk_state_to_plex(user_id: int, changes: list[tuple[int, str, str]]) -
 
 # --- Phase C: incremental + full Plex ↔ Siftlode reconcile (scheduler jobs) ------------------------
 
-# Clock-skew / round-trip slack when comparing a Plex timestamp to a Siftlode one. A state we just
-# pushed (Plex stamps viewedAt a beat after our watched_at) must not read as "newer on Plex" and get
-# pulled back with a different shape. Genuine Plex-side edits are minutes from our writes — far
-# outside this — so the window only absorbs skew, never real changes.
-_LWW_TOLERANCE_S = 90
-
 
 def _active_sync_links(db: Session) -> list[PlexLink]:
     """Every link with two-way sync actually running (owner MVP = one row). Iterated uniformly so
@@ -338,11 +335,6 @@ def _resolve_account_id(db: Session, plex: PlexClient, link: PlexLink) -> int:
     return acct_id
 
 
-def _sift_ts(st: PlexState) -> datetime | None:
-    """The freshest Siftlode-side timestamp on a row, for last-write-wins."""
-    return max([t for t in (st.watched_at, st.progress_updated_at) if t], default=None)
-
-
 def _same_state(st: PlexState | None, status: str, pos: int) -> bool:
     """Whether a row already matches a target (status, pos) — the core ping-pong guard: when nothing
     would change we skip the write entirely (no row churn, no timestamp bump, no re-pull)."""
@@ -370,10 +362,13 @@ def _pull_apply(
         if st is not None and not st.synced_to_plex:
             st.synced_to_plex = True  # already matches Plex → just settle the flag, no churn
         return None
-    # Conflict: keep Siftlode when it's newer than Plex (plus skew slack).
-    sts = _sift_ts(st) if st else None
-    if sts and plex_ts and plex_ts <= sts + timedelta(seconds=_LWW_TOLERANCE_S):
-        return None
+    # Plex-authoritative (S2): on a genuine divergence for an item Plex knows about, Plex wins — no
+    # last-write-wins tug-of-war. This is safe because prompt write-back (playback.py pushes the resume
+    # position to Plex on the ~10s checkpoint, not just on the final flush) keeps Plex current, so a
+    # local edit that's "newer" than Plex has almost always already been pushed and matched above by
+    # `_same_state`; a row that reaches here really is out of step with the source of truth. The
+    # `_same_state` + `synced_to_plex` short-circuit above still absorbs the freshly-pushed case (Plex
+    # echoes the same shape a beat later) so a just-pushed row never bounces.
     if st is None:
         st = PlexState(user_id=user_id, item_id=item_id)
         db.add(st)