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/requirements.txt

@@ -7,5 +7,7 @@ pydantic>=2.7,<3.0
 pydantic-settings>=2.3,<3.0
 authlib>=1.3,<2.0
 httpx>=0.27,<1.0
+feedparser>=6.0,<7.0
+apscheduler>=3.10,<4.0
 itsdangerous>=2.1,<3.0
 cryptography>=42,<46