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

PlexLibrary, PlexShow, PlexSeason, PlexItem, PlexPerson, PlexCollection, PlexState, PlexLink,
PlexPlaylist and PlexPlaylistItem leave app/models.py for app/plex/models.py; app.tables registers
them. Every import is re-pointed; no schema change.

backend/tests/test_plex_sync_prune.py

@@ -8,14 +8,8 @@ with a stub Plex (the same seam the overlay test uses), so no live server is nee
 
 from sqlalchemy import select
 
-from app.models import (
-    PlexItem,
-    PlexLibrary,
-    PlexSeason,
-    PlexShow,
-    PlexState,
-    User,
-)
+from app.models import User
+from app.plex.models import PlexItem, PlexLibrary, PlexSeason, PlexShow, PlexState
 from app.plex.sync import (
     _TYPE_EPISODE,
     _TYPE_MOVIE,