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_downloads_finalize.py

@@ -11,7 +11,8 @@ from sqlalchemy import text
 
 from app.downloads import service
 from app.downloads.status import JobStatus
-from app.models import DownloadJob, User
+from app.models import User
+from app.downloads.models import DownloadJob
 
 
 def _running_job(db) -> DownloadJob: