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/scheduler.py

@@ -15,7 +15,8 @@ from app import audit, debugsession, jobruns, logs, logsink, progress, quota, sy
 from app.config import settings
 from app.db import SessionLocal
 from app.downloads.gc import run_download_gc
-from app.models import Channel, JobRun
+from app.models import JobRun
+from app.catalog.models import Channel
 from app.plex.sync import sync as run_plex_sync
 from app.plex.tmdb import run_enrich as run_tmdb_enrich
 from app.plex.tmdb import run_people_enrich