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_clip.py

@@ -18,7 +18,7 @@ from app.downloads import errors as dlerrors
 from app.downloads import formats, pipeline, quota, runner, service
 from app.downloads import probe as probemod
 from app.downloads.edit import INPUT_GUARD
-from app.models import PlexItem, PlexLibrary, PlexShow
+from app.plex.models import PlexItem, PlexLibrary, PlexShow
 from app.catalog.models import Channel, Video
 from app.downloads.models import DownloadJob, MediaAsset
 from app.plex import paths as plex_paths