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/sync.py

@@ -6,7 +6,8 @@ from app import audit, quota, state
 from app.audit import AuditAction
 from app.auth import admin_user, current_user, has_read_scope
 from app.db import get_db
-from app.models import Channel, Subscription, User, Video
+from app.models import User
+from app.catalog.models import Channel, Subscription, Video
 from app.routes._common import youtube_refusal
 from app.ratelimit import RateLimiter
 from app.sync.runner import estimate_deep_backfill