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/routes/tags.py

@@ -0,0 +1,43 @@
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy import func, or_, select
+from sqlalchemy.orm import Session
+
+from app.auth import current_user
+from app.db import get_db
+from app.models import ChannelTag, Tag, User
+from app.sync.autotag import run_autotag_all
+
+router = APIRouter(prefix="/api/tags", tags=["tags"])
+
+
+@router.get("")
+def list_tags(user: User = Depends(current_user), db: Session = Depends(get_db)) -> list[dict]:
+    rows = db.execute(
+        select(Tag, func.count(ChannelTag.id))
+        .outerjoin(ChannelTag, ChannelTag.tag_id == Tag.id)
+        .where(or_(Tag.user_id.is_(None), Tag.user_id == user.id))
+        .group_by(Tag.id)
+        .order_by(Tag.category, Tag.name)
+    ).all()
+    return [
+        {
+            "id": tag.id,
+            "name": tag.name,
+            "color": tag.color,
+            "category": tag.category,
+            "system": tag.user_id is None,
+            "channel_count": count,
+        }
+        for tag, count in rows
+    ]
+
+
+@router.post("/recompute")
+def recompute(
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+    only_missing: bool = False,
+) -> dict:
+    if user.role != "admin":
+        raise HTTPException(status_code=403, detail="Admin only")
+    return run_autotag_all(db, only_missing=only_missing)