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/youtube/rss.py

@@ -8,14 +8,24 @@ import httpx
 RSS_URL = "https://www.youtube.com/feeds/videos.xml?channel_id={channel_id}"
 
 
+class RssError(Exception):
+    """The feed could not be read (transport failure or a non-200 answer).
+
+    Deliberately NOT the same as an empty feed: a failed poll used to return `[]`, which the
+    caller stamped as a successful `last_rss_at` — so a channel whose feed 404s permanently
+    (deleted, renamed id) looked like it was being polled fine forever, and nothing surfaced it."""
+
+
 def fetch_channel_feed(channel_id: str) -> list[dict]:
+    """The channel's recent uploads as stubs. `[]` means the feed really is empty; a failure
+    raises RssError so the caller can leave the channel un-stamped and log it."""
     url = RSS_URL.format(channel_id=channel_id)
     try:
         resp = httpx.get(url, timeout=20.0, headers={"User-Agent": "Siftlode/1.0"})
-    except httpx.HTTPError:
-        return []
+    except httpx.HTTPError as exc:
+        raise RssError(f"RSS fetch failed for {channel_id}: {exc}") from exc
     if resp.status_code != 200:
-        return []
+        raise RssError(f"RSS fetch for {channel_id} returned HTTP {resp.status_code}")
 
     parsed = feedparser.parse(resp.content)
     out: list[dict] = []