fix(plex): a checkpoint sent just before the finish no longer lands on the watched row

The 10 s tick in flight when the player finishes (or a refusal rollback whose state write in fact
committed) wrote its mid-item position onto the freshly watched row and scheduled a resume push
after the scrobble, so Plex showed the item in progress again. The checkpoint is now a conditional
write that skips a row marked watched in the last 30 s and answers like a finish; a rewatch past
that is checkpointed as before. Marking an item (or a season/show) watched also supersedes resume
pushes still in flight, as the near-end finish already did.

backend/app/routes/plex/playback.py

@@ -234,6 +234,15 @@ def person_image(u: str, request: Request) -> Response:
     return imgcache.response(data, ct)
 
 
+def _supersede_resume_pushes(user_id: int, item_id: int, now: datetime) -> None:
+    """A watched mark supersedes every resume push still in flight for the item: stamping it as the
+    latest push makes `_resume_push_superseded` drop them. Popping the entry instead (to keep the map
+    small) removed the very guard, and an older resume push could then land at Plex AFTER the
+    scrobble and revive a resume point on a watched item. Eviction bounds the map."""
+    with _push_throttle_lock:
+        _last_push_at[(user_id, item_id)] = now
+
+
 def _push_resume_guarded(
     user_id: int,
     item_id: int,
@@ -329,12 +338,7 @@ def item_progress(
     near_end = duration > 0 and position > duration - _FINISH_MARGIN_S
     if near_end:
         flipped = service.mark_finished(db, user.id, it.id, now)
-        # The finish supersedes every resume push still in flight for this item: stamping it as the
-        # latest push makes `_resume_push_superseded` drop them. Popping the entry instead (to keep
-        # the map small) removed the very guard, and an older resume push could then land at Plex
-        # AFTER the scrobble and revive a resume point on a watched item. Eviction bounds the map.
-        with _push_throttle_lock:
-            _last_push_at[(user.id, it.id)] = now
+        _supersede_resume_pushes(user.id, it.id, now)
         db.commit()
         # Only the checkpoint that flipped the row scrobbles: a later checkpoint of the same finished
         # item — or a second one racing this one — would otherwise bump Plex's viewCount again.
@@ -352,7 +356,11 @@ def item_progress(
     push_now = _should_push_resume(user.id, it.id, now, final)
     # Marked dirty before the (background) push so the reconcile knows to re-push if it fails, and so
     # `push_state_to_plex` can settle synced_to_plex once Plex confirms.
-    service.checkpoint(db, user, it.id, st, position, now, dirty=push_now)
+    if not service.checkpoint(db, user, it.id, st, position, now, dirty=push_now):
+        # Marked watched a moment ago: this is the finishing player's late tick (see
+        # `service.checkpoint`), and the answer is the same one a near-end checkpoint gets.
+        db.commit()
+        return {"ok": True, "status": "watched", "position_seconds": 0}
     db.commit()
     if push_now:
         _push_watch(
@@ -408,6 +416,8 @@ def item_state(
             _push_watch(background, db, user.id, it.id, rating_key, "unwatched")
         return {"ok": True, "status": "new"}
     service.set_status(db, user, it.id, st, status)
+    if status == "watched":
+        _supersede_resume_pushes(user.id, it.id, datetime.now(timezone.utc))
     db.commit()
     if status == "watched" and prev_status != "watched":
         _push_watch(background, db, user.id, it.id, rating_key, "watched")
@@ -427,6 +437,10 @@ def _bulk_state(
     always reads the committed marks."""
     pushable = plex_watch.link_for_push(db, user_id) is not None
     to_push = service.bulk_state(db, user_id, eps, watched)
+    if watched:
+        now = datetime.now(timezone.utc)
+        for item_id, _rating_key, _action in to_push:
+            _supersede_resume_pushes(user_id, item_id, now)
     if pushable and to_push:
         # One coalesced background task for the whole batch (reuses a single session + Plex client),
         # not one task per episode — a 200-ep show mark is 1 task, not 200.