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_collation.py

@@ -9,7 +9,8 @@ from app.auth import current_user
 from app.collation import sorted_words
 from app.db import get_db
 from app.main import app
-from app.models import Channel, PlexCollection, PlexItem, PlexLibrary, Subscription, User, Video
+from app.models import PlexCollection, PlexItem, PlexLibrary, User
+from app.catalog.models import Channel, Subscription, Video
 from app.reader.models import Playlist, PlaylistItem
 from datetime import datetime, timezone
 
@@ -222,7 +223,7 @@ def test_the_share_recipients_are_in_the_readers_alphabet(db, monkeypatch, overr
 
 
 def test_the_blocked_channels_are_in_the_readers_alphabet(db, monkeypatch, override_dep):
-    from app.models import BlockedChannel
+    from app.catalog.models import BlockedChannel
 
     user = _user(db, "hu")
     db.add_all(Channel(id=f"UC{i}", title=w) for i, w in enumerate(WORDS))