fix(sync): R5 S2 — sync guards

- never prune subscriptions on an empty fetch: one anomalous empty-but-200
  response wiped every subscription (and its deep_requested flag) in one run
- push_playlist: stop after 5 consecutive write failures — a quota-dead or
  scope-revoked account otherwise burns 50 units per remaining item
- RSS: raise RssError instead of returning [] on a transport/HTTP failure, so a
  failed poll no longer stamps last_rss_at and a 404 feed stops looking healthy
- worker: exit when the schema never appears instead of "continuing" into a
  crash one call later

backend/app/sync/playlists.py

@@ -263,6 +263,40 @@ def plan_push(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
     }
 
 
+# After this many consecutive write failures we stop calling YouTube for the rest of the push.
+# The failure modes that matter here are all account-wide, not per-video: quota exhausted, the
+# write scope revoked, the token dead. Every remaining write then fails identically — but each
+# one still SPENDS 50 units, so a long playlist burns through the whole (already dead) budget,
+# and tomorrow's too, to produce a failure list we knew after the fifth item.
+_MAX_CONSECUTIVE_FAILURES = 5
+
+
+class _Breaker:
+    """Consecutive-failure circuit breaker for a single push run. Any success re-arms it."""
+
+    def __init__(self, limit: int = _MAX_CONSECUTIVE_FAILURES) -> None:
+        self.limit = limit
+        self.streak = 0
+        self.tripped = False
+
+    def open(self) -> bool:
+        """True while writes may still be attempted."""
+        return not self.tripped
+
+    def hit(self, ok: bool) -> None:
+        if ok:
+            self.streak = 0
+            return
+        self.streak += 1
+        if self.streak >= self.limit and not self.tripped:
+            self.tripped = True
+            log.warning(
+                "playlist push: %d consecutive YouTube write failures — stopping this run "
+                "(quota, scope or token); the remaining items are reported as failures",
+                self.limit,
+            )
+
+
 def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
     """Push a local playlist to YouTube: create it if needed, then reconcile membership
     and order so YouTube matches local (local wins). Marks the playlist clean on success.
@@ -271,6 +305,7 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
     desired = _local_video_ids(db, pl)
     inserted = deleted = reordered = created = 0
     failures: list[str] = []
+    breaker = _Breaker()
     with YouTubeClient(db, user) as yt:
         if not pl.yt_playlist_id:
             pl.yt_playlist_id = yt.create_playlist(pl.name)
@@ -278,11 +313,16 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
             created = 1
             # Brand-new playlist: just append every video in order.
             for vid in desired:
+                if not breaker.open():
+                    failures.append(vid)
+                    continue
                 try:
                     yt.add_playlist_item(pl.yt_playlist_id, vid)
                     inserted += 1
+                    breaker.hit(True)
                 except YouTubeError:
                     failures.append(vid)
+                    breaker.hit(False)
             if not failures:
                 pl.synced_fingerprint = fingerprint(pl.name, desired)
                 pl.dirty = False
@@ -293,6 +333,7 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
                 "deleted": deleted,
                 "reordered": reordered,
                 "failures": failures,
+                "aborted": breaker.tripped,
                 "yt_playlist_id": pl.yt_playlist_id,
             }
 
@@ -313,20 +354,30 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
         # Delete extras (local is authoritative).
         for it in current:
             if it["video_id"] not in desired_set:
+                if not breaker.open():
+                    failures.append(it["video_id"])
+                    continue
                 try:
                     yt.delete_playlist_item(it["item_id"])
                     deleted += 1
+                    breaker.hit(True)
                 except YouTubeError:
                     failures.append(it["video_id"])
+                    breaker.hit(False)
         # Insert missing (append; the reorder pass below fixes positions).
         cur_set = set(cur_vids)
         for vid in desired:
             if vid not in cur_set:
+                if not breaker.open():
+                    failures.append(vid)
+                    continue
                 try:
                     item_id_by_vid[vid] = yt.add_playlist_item(pl.yt_playlist_id, vid)
                     inserted += 1
+                    breaker.hit(True)
                 except YouTubeError:
                     failures.append(vid)
+                    breaker.hit(False)
         # Reorder to match `desired` via insertion-sort moves over a local model.
         model = [v for v in cur_vids if v in desired_set] + [
             v for v in desired if v not in cur_set and v in item_id_by_vid
@@ -336,11 +387,16 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
                 continue  # failed insert; skip
             if i < len(model) and model[i] == want:
                 continue
+            if not breaker.open():
+                failures.append(want)
+                continue
             try:
                 yt.move_playlist_item(item_id_by_vid[want], pl.yt_playlist_id, want, i)
                 reordered += 1
+                breaker.hit(True)
             except YouTubeError:
                 failures.append(want)
+                breaker.hit(False)
                 continue
             j = model.index(want)
             model.pop(j)
@@ -355,6 +411,7 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
         "deleted": deleted,
         "reordered": reordered,
         "failures": failures,
+        "aborted": breaker.tripped,
         "yt_playlist_id": pl.yt_playlist_id,
     }