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_jsonb_normalize.py

@@ -7,7 +7,7 @@ Uses the DB lane (`db` fixture) — skipped when no Postgres is reachable.
 
 from sqlalchemy import text
 
-from app.models import PlexLibrary, PlexShow
+from app.plex.models import PlexLibrary, PlexShow
 
 # Mirror of migration 0060's statements (plex_items runs against an empty table here — proving the
 # SQL is valid for both tables; the assertions below check the seeded plex_shows rows).