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/app/routes/plex/_shared.py

@@ -3,7 +3,7 @@
 from sqlalchemy.orm import Session
 
 from app import logs
-from app.models import PlexItem
+from app.plex.models import PlexItem
 from app.plex.cards import norm_lang as _norm_lang  # noqa: F401 — the package's historic name
 from app.plex.lookups import by_rating_key
 from app.routes._common import not_found