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/routes/public.py

@@ -17,7 +17,8 @@ from app.db import get_db
 from app.downloads import links as linksmod
 from app.downloads import storage
 from app.downloads.status import JobStatus
-from app.models import DownloadJob, DownloadLink, FileFolder, MediaAsset, User
+from app.models import User
+from app.downloads.models import DownloadJob, DownloadLink, FileFolder, MediaAsset
 from app.ratelimit import RateLimiter, client_key
 from app.security import verify_password
 from app.refusal import refused