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_clip.py

@@ -18,7 +18,8 @@ from app.downloads import errors as dlerrors
 from app.downloads import formats, pipeline, quota, runner, service
 from app.downloads import probe as probemod
 from app.downloads.edit import INPUT_GUARD
-from app.models import Channel, PlexItem, PlexLibrary, PlexShow, Video
+from app.models import PlexItem, PlexLibrary, PlexShow
+from app.catalog.models import Channel, Video
 from app.downloads.models import DownloadJob, MediaAsset
 from app.plex import paths as plex_paths