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_query_conversion.py

@@ -16,7 +16,8 @@ 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 rollup, sync
 from app.plex.cards import ShowState
 
@@ -490,7 +491,7 @@ class TestPlaylistAndCollectionEndpoints:
         ]  # input order preserved, m2 gone
 
     def test_collections_list(self, db, monkeypatch, override_dep):
-        from app.models import PlexCollection
+        from app.plex.models import PlexCollection
 
         lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
         db.add(lib)