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_activity_reads.py

@@ -9,7 +9,8 @@ from datetime import datetime, timedelta, timezone
 
 from app.auth import admin_user
 from app.main import app
-from app.models import AuditLog, Channel, JobRun, LogEvent, QuotaEvent
+from app.models import AuditLog, JobRun, LogEvent, QuotaEvent
+from app.catalog.models import Channel
 from tests.test_body_limit import _route_objects
 
 T0 = datetime(2026, 10, 1, 12, 0, tzinfo=timezone.utc)