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_discover.py

@@ -17,17 +17,8 @@ from app.discover import hero, pins, queries, rails
 from app.discover.budget import BANDS, fits, remaining_seconds
 from app.plex import rollup
 from app.plex.cards import leaf_card
-from app.models import (
-    BlockedChannel,
-    Channel,
-    PlexItem,
-    PlexLibrary,
-    PlexShow,
-    PlexState,
-    Subscription,
-    User,
-    Video,
-)
+from app.models import PlexItem, PlexLibrary, PlexShow, PlexState, User
+from app.catalog.models import BlockedChannel, Channel, Subscription, Video
 from app.reader.models import ComfortPin, Playlist, PlaylistItem, VideoState
 
 NOW = datetime(2026, 6, 1, tzinfo=timezone.utc)