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/downloads/service.py

@@ -24,7 +24,8 @@ from app.downloads import credentials
 from app.downloads import edit as editmod
 from app.downloads import folders, formats, mediakind, quota, storage
 from app.downloads.status import HOLDING, JobKind, JobStatus
-from app.models import DISPLAY_TEXT_MAX, Channel, PlexItem, PlexShow, User, Video
+from app.models import DISPLAY_TEXT_MAX, PlexItem, PlexShow, User
+from app.catalog.models import Channel, Video
 from app.downloads.models import (
     DownloadJob,
     DownloadProfile,