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_abuse_limits.py

@@ -106,7 +106,7 @@ def test_the_single_playlist_repull_shares_the_pull_cooldown(db, as_user, make_u
     # SEC-14, the sweep half: `revert-youtube` spends under the SAME QuotaAction.PLAYLISTS_PULL key
     # as the bulk `sync-youtube`, so limiting only one of them left the identical drain open one
     # route over. Both must draw on the one allowance.
-    from app.models import Playlist
+    from app.reader.models import Playlist
     from app.routes import playlists as plroutes
 
     user = make_user()