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_detail.py

@@ -19,7 +19,8 @@ import app.state as state
 from app.auth import current_user
 from app.db import get_db
 from app.main import app
-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 import detail
 from app.plex.sync import _apply_facet_fields, _enrich_people_batch