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/tests/test_files_api.py

@@ -18,7 +18,8 @@ from app.config import settings
 from app.downloads import folders, service
 from app.downloads import upload as uploadmod
 from app.downloads.status import JobStatus
-from app.models import AuditLog, DownloadJob, DownloadShare, FileFolder, MediaAsset
+from app.models import AuditLog
+from app.downloads.models import DownloadJob, DownloadShare, FileFolder, MediaAsset
 
 NOW = datetime.now(timezone.utc)