refactor(capture): the unit's tables move into downloads/models

The nine Capture tables (files, jobs, folders, shares, links, quota, cookies, profiles, library
resume points) leave app/models.py for the unit's own module; app.tables registers every unit's
tables for the migrations, the test database and the ownership check. No schema change.

backend/app/downloads/upload.py

@@ -37,7 +37,8 @@ from app.config import settings
 from app.downloads import edit as editmod
 from app.downloads import folders, mediakind, playable, probe, quota, service, storage
 from app.downloads.status import JobKind, JobStatus
-from app.models import DownloadJob, MediaAsset, display_text
+from app.models import display_text
+from app.downloads.models import DownloadJob, MediaAsset
 
 
 def _title_from_filename(filename: str) -> str: