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/runner.py

@@ -25,7 +25,7 @@ from app.sync.videos import (
     run_shorts_classification,
 )
 from app.youtube.client import YouTubeClient
-from app.youtube.rss import fetch_channel_feed
+from app.youtube.rss import RssError, fetch_channel_feed
 
 # RSS feeds are free and network-bound, so fetch many at once; DB writes stay single-threaded.
 RSS_POLL_WORKERS = 16
@@ -54,6 +54,7 @@ def run_rss_poll(db: Session, channels: list[Channel] | None = None) -> int:
     total = len(channels)
     new = 0
     done = 0
+    failed = 0
     # Fetch feeds concurrently (network-bound, no DB), then apply inserts on this thread as
     # each fetch returns — a SQLAlchemy session isn't thread-safe. Pass the channel id (a
     # plain str) to the workers; the ORM instance is touched only here on the main thread.
@@ -65,9 +66,20 @@ def run_rss_poll(db: Session, channels: list[Channel] | None = None) -> int:
             channel = futures[fut]
             try:
                 entries = fut.result()
+            except RssError as exc:
+                # A failed poll is NOT an empty feed: skip the apply so `last_rss_at` stays where
+                # it was. Stamping it here made a permanently-broken feed look freshly polled.
+                log.warning("RSS poll skipped for channel %s: %s", channel.id, exc)
+                failed += 1
+                done += 1
+                progress.report(done, total, "rss_poll")
+                continue
             except Exception:
                 log.exception("RSS fetch failed for channel %s", channel.id)
-                entries = []
+                failed += 1
+                done += 1
+                progress.report(done, total, "rss_poll")
+                continue
             try:
                 new += apply_rss_feed(db, channel, entries)
             except Exception:
@@ -75,6 +87,8 @@ def run_rss_poll(db: Session, channels: list[Channel] | None = None) -> int:
                 log.exception("RSS apply failed for channel %s", channel.id)
             done += 1
             progress.report(done, total, "rss_poll")
+    if failed:
+        log.warning("RSS poll: %d/%d channel feeds could not be read", failed, total)
     return new