feat: M2 (part 2) — RSS poller, backfill, enrichment, scheduler

- Free per-channel RSS reader for quota-less fresh-video detection
- Recent-first backfill (configurable: 100 videos / 1 year) plus resumable deep
  backfill from the uploads playlist
- Enrichment via videos.list: duration, view/like counts, category, topics,
  language, Shorts heuristic and livestream/premiere classification
- Reusable sync runners + APScheduler jobs (rss / enrich / backfill), all
  quota-aware with a reserve so backfill never starves fresh enrichment
- Manual triggers: POST /api/sync/{rss,backfill,enrich}
- Exact insert counting via RETURNING with in-batch de-duplication

backend/app/config.py

@@ -31,9 +31,23 @@ class Settings(BaseSettings):
     # Daily quota budget (units). Default leaves headroom under the 10,000/day free limit.
     quota_daily_budget: int = 9000
     # Recent-first backfill: how far back to fetch on the first pass per channel.
-    backfill_recent_max_videos: int = 200
+    backfill_recent_max_videos: int = 100
     backfill_recent_max_days: int = 365
 
+    # Videos at or below this duration (seconds) are treated as Shorts.
+    shorts_max_seconds: int = 60
+    # videos.list accepts up to 50 ids per call.
+    enrich_batch_size: int = 50
+
+    # --- Background scheduler ---
+    scheduler_enabled: bool = True
+    rss_poll_minutes: int = 20
+    enrich_interval_minutes: int = 3
+    backfill_interval_minutes: int = 10
+    # Keep this many quota units in reserve so scheduled backfill never starves
+    # interactive syncs / enrichment of fresh videos.
+    backfill_quota_reserve: int = 2000
+
     @property
     def allowed_email_set(self) -> set[str]:
         return {e.strip().lower() for e in self.allowed_emails.split(",") if e.strip()}