feat: M3 — automatic channel tagging (language + topic system tags)

- Tag and ChannelTag models + migration 0003 (partial unique indexes split
  system vs per-user tag names)
- Offline language detection (py3langid) constrained to a curated language set,
  with the channel's declared default language as a strong prior
- Topic tags mapped from YouTube topicDetails + dominant video category; the
  generic "Lifestyle" catch-all is intentionally dropped
- System (auto) tags are regenerated idempotently and never touch user tags;
  orphaned system tags are cleaned up
- GET /api/tags and admin POST /api/tags/recompute; scheduled autotag job

backend/requirements.txt

@@ -9,5 +9,6 @@ authlib>=1.3,<2.0
 httpx>=0.27,<1.0
 feedparser>=6.0,<7.0
 apscheduler>=3.10,<4.0
+py3langid>=0.3,<1.0
 itsdangerous>=2.1,<3.0
 cryptography>=42,<46