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_link_token_at_rest.py

@@ -18,7 +18,8 @@ from app import security
 from app.config import settings
 from app.db import engine
 from app.downloads import probe as probemod
-from app.models import AuditLog, DownloadLink
+from app.models import AuditLog
+from app.downloads.models import DownloadLink
 
 _VIDEO = {
     "streams": [