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/app/sync/playlists.py

@@ -21,7 +21,8 @@ from sqlalchemy.pool import NullPool
 
 from app import logs, progress, quota
 from app.auth import has_read_scope
-from app.models import Channel, Playlist, PlaylistItem, User, Video
+from app.models import Channel, User, Video
+from app.reader.models import Playlist, PlaylistItem
 from app.sync.playlist_merge import merge_plan
 from app.sync.runner import token_users
 from app.sync.subscriptions import blank_listing, refuse_blank_listing, vanished