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/routes/runs.py

@@ -18,7 +18,8 @@ from sqlalchemy.orm import Session
 from app import scheduler
 from app.auth import admin_user
 from app.db import get_db
-from app.models import Channel, JobRun, LogEvent, User
+from app.models import JobRun, LogEvent, User
+from app.catalog.models import Channel
 from app.routes import _account as account
 from app.routes._keyset import PAGE_MAX, accounts, older_than, page, utc
 from app.runstatus import RunStatus