refactor(reader): the unit's tables move into reader/models

VideoState, Playlist, PlaylistItem, SavedView, ComfortPin, Tag and ChannelTag leave app/models.py
for app/reader/models.py; app.tables registers them. No schema change.

backend/tests/test_discover.py

@@ -20,18 +20,15 @@ from app.plex.cards import leaf_card
 from app.models import (
     BlockedChannel,
     Channel,
-    ComfortPin,
     PlexItem,
     PlexLibrary,
     PlexShow,
     PlexState,
-    Playlist,
-    PlaylistItem,
     Subscription,
     User,
     Video,
-    VideoState,
 )
+from app.reader.models import ComfortPin, Playlist, PlaylistItem, VideoState
 
 NOW = datetime(2026, 6, 1, tzinfo=timezone.utc)