refactor(catalog): the unit's tables move into catalog/models

Channel, Subscription, Video, SearchFind, ExploredChannel and BlockedChannel (and LIVE_OR_UPCOMING)
leave app/models.py for app/catalog/models.py; app.tables registers them. The User↔Subscription
relationship names its other side as a type only. No schema change.

backend/app/sync/videos.py

@@ -12,7 +12,7 @@ from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session
 
 from app import logs, progress, quota, sysconfig
-from app.models import LIVE_OR_UPCOMING, Channel, Video
+from app.catalog.models import LIVE_OR_UPCOMING, Channel, Video
 from app.titles import normalize_title
 from app.youtube.client import YouTubeClient, YouTubeError, best_thumbnail, is_playlist_gone
 from app.youtube.shorts import make_client, probe_is_short