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_subtitle_prepare.py

@@ -1088,7 +1088,7 @@ def test_a_nul_in_the_rating_key_is_not_a_500(client, monkeypatch):
 
 def test_a_nul_rating_key_names_no_row():
     """Postgres refuses NUL in text, so looking it up was a DataError (a 500) on every Plex route."""
-    from app.models import PlexItem
+    from app.plex.models import PlexItem
     from app.plex.lookups import by_rating_key
 
     assert by_rating_key(None, PlexItem, "12\x0034") is None  # type: ignore[arg-type]