refactor(catalog): the HTTP layer reads through queries and writes through the service

Channels, search and sync do no database work in their routes any more: reads go through
catalog/queries, writes through catalog/service, and a live search's ingest is catalog/search.
What a catalog route needs from Reader (a playlist's channels, a user's tag links, the feed
projection of results) it asks reader/queries and reader/service. The scheduler and the runs page
read channel titles and the work queue through catalog/queries. Catalog is fenced below its HTTP
layer: its owner filters go through userscope, it reads no other unit's tables, and its ingest
jobs commit only where they are named.

backend/app/reader/service.py

@@ -464,3 +464,17 @@ def reorder_items(db: Session, pl: Playlist, order: list[str]) -> int:
     touch(pl)
     db.flush()
     return pos
+
+
+def attach_user_tag(db: Session, user: User, channel_id: str, tag_id: int) -> None:
+    """The user's own label on a channel (idempotent). The caller has checked the tag is theirs."""
+    if queries.user_tag_link(db, user, channel_id, tag_id) is None:
+        db.add(ChannelTag(channel_id=channel_id, tag_id=tag_id, user_id=user.id, source="user"))
+        db.flush()
+
+
+def detach_user_tag(db: Session, user: User, channel_id: str, tag_id: int) -> None:
+    row = queries.user_tag_link(db, user, channel_id, tag_id)
+    if row is not None:
+        db.delete(row)
+        db.flush()