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_facets_grid_parity.py

@@ -15,14 +15,8 @@ the merge is provably behaviour-equivalent (and a drift between grid and facets
 from collections import Counter
 from datetime import datetime, timezone
 
-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
 
 SCOPES = ("movie", "show", "both")
 SHOW_STATES = ("all", "watched", "in_progress", "unwatched")