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/tests/test_scheduler.py

@@ -13,7 +13,8 @@ import pytest
 from sqlalchemy import select
 
 from app import jobruns, logs, logsink, quota, scheduler, state, sysconfig
-from app.models import AuditLog, Channel, JobRun, Notification, QuotaEvent
+from app.models import AuditLog, JobRun, Notification, QuotaEvent
+from app.catalog.models import Channel
 from app.routes import scheduler as scheduler_routes
 from app.runstatus import RunStatus, RunTrigger