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

README.md

@@ -15,6 +15,9 @@ once and stored locally, so filtering/searching/sorting are instant and don't bu
 > - **M2** (ingest core): subscription import, free RSS detection, recent-first + deep backfill
 >   from the uploads playlist, enrichment (duration/stats/category, Shorts & livestream
 >   classification), a shared daily quota guard, and a background scheduler.
+> - **M3** (auto-tagging): system tags for channel language (offline detection) and topic
+>   (from YouTube topics + dominant category), regenerated automatically; user tags are
+>   never overwritten.
 
 ## Requirements