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/subscriptions.py

@@ -108,17 +108,29 @@ def import_subscriptions(db: Session, user: User) -> dict:
             sub_row.yt_subscription_id = sub.get("yt_subscription_id")
         db.flush()
 
-        # Prune subscriptions removed on YouTube.
+        # Prune subscriptions removed on YouTube — but NEVER on an empty fetch. The API can answer
+        # 200 with no items (a transient blip, a scope/token edge), and pruning against that wipes
+        # every subscription the user has, taking each row's `deep_requested` flag with it, in one
+        # run. Someone who genuinely unsubscribed from everything simply keeps their rows until a
+        # sync that returns something.
         removed = 0
         existing = (
             db.execute(select(Subscription).where(Subscription.user_id == user.id))
             .scalars()
             .all()
         )
-        for sub_row in existing:
-            if sub_row.channel_id not in fetched:
-                db.delete(sub_row)
-                removed += 1
+        prune_skipped = bool(existing) and not fetched
+        if prune_skipped:
+            log.warning(
+                "Subscription import (user %s): YouTube returned 0 subscriptions while %d exist "
+                "locally — skipping the prune (treating it as a transient empty response)",
+                user.id, len(existing),
+            )
+        else:
+            for sub_row in existing:
+                if sub_row.channel_id not in fetched:
+                    db.delete(sub_row)
+                    removed += 1
         db.commit()
 
         # Fetch details for channels that don't have them yet.
@@ -144,6 +156,7 @@ def import_subscriptions(db: Session, user: User) -> dict:
         "channels_new": new_channels,
         "channels_detailed": detailed,
         "removed_stale": removed,
+        "prune_skipped": prune_skipped,
     }
     log.info("Subscription import (user %s): %s", user.id, result)
     return result