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/app/config.py

@@ -48,6 +48,14 @@ class Settings(BaseSettings):
     # interactive syncs / enrichment of fresh videos.
     backfill_quota_reserve: int = 2000
 
+    # --- Auto-tagging / feed defaults ---
+    # Number of recent video titles sampled per channel for language detection.
+    autotag_title_sample: int = 40
+    autotag_interval_minutes: int = 30
+    # live_status values hidden from the feed by default. Completed-stream VODs
+    # ("was_live") are real watchable content and stay visible.
+    feed_default_hidden_live: str = "live,upcoming"
+
     @property
     def allowed_email_set(self) -> set[str]:
         return {e.strip().lower() for e in self.allowed_emails.split(",") if e.strip()}