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/downloads/pipeline.py

@@ -28,7 +28,8 @@ from app.downloads.jobdb import (
 )
 from app.downloads.stated import StatedFailure
 from app.downloads.status import JobStatus
-from app.models import DISPLAY_TEXT_MAX, PlexItem
+from app.models import DISPLAY_TEXT_MAX
+from app.plex.models import PlexItem
 from app.downloads.models import DownloadJob, MediaAsset
 from app.plex import lookups as plex_lookups
 from app.plex import paths as plex_paths

backend/app/downloads/service.py

@@ -24,7 +24,8 @@ from app.downloads import credentials
 from app.downloads import edit as editmod
 from app.downloads import folders, formats, mediakind, quota, storage
 from app.downloads.status import HOLDING, JobKind, JobStatus
-from app.models import DISPLAY_TEXT_MAX, PlexItem, PlexShow, User
+from app.models import DISPLAY_TEXT_MAX, User
+from app.plex.models import PlexItem, PlexShow
 from app.catalog.models import Channel, Video
 from app.downloads.models import (
     DownloadJob,