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_playlist_add_position.py

@@ -8,7 +8,8 @@ the position is an index over the ORDERED rows, and the list is renumbered aroun
 
 from datetime import datetime, timezone
 
-from app.models import Channel, Playlist, PlaylistItem, Video
+from app.models import Channel, Video
+from app.reader.models import Playlist, PlaylistItem
 
 
 def _seed(db, user, ids, *, positions=None, **kw):