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/routes/channels.py

@@ -19,15 +19,12 @@ from app.models import (
     LIVE_OR_UPCOMING,
     BlockedChannel,
     Channel,
-    ChannelTag,
     ExploredChannel,
-    Playlist,
-    PlaylistItem,
     Subscription,
-    Tag,
     User,
     Video,
 )
+from app.reader.models import ChannelTag, Playlist, PlaylistItem, Tag
 from app.routes._common import get_or_404, not_found, owned_or_404, youtube_refusal
 from app.sync.explore import explore_ingest_page
 from app.sync.runner import backfill_channel_now, run_recent_backfill