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_folder_links.py

@@ -18,7 +18,8 @@ from app.config import settings
 from app.db import engine
 from app.downloads import folders, og
 from app.downloads import probe as probemod
-from app.models import AuditLog, DownloadJob, DownloadLink, FileFolder, MediaAsset
+from app.models import AuditLog
+from app.downloads.models import DownloadJob, DownloadLink, FileFolder, MediaAsset
 
 _VIDEO = {
     "streams": [
@@ -317,7 +318,7 @@ def test_a_password_file_link_says_it_opens_a_file(shelf, client):
 def test_a_file_whose_bytes_are_gone_is_not_offered(shelf, client, db):
     from pathlib import Path
 
-    from app.models import MediaAsset
+    from app.downloads.models import MediaAsset
 
     token = _link(shelf)["token"]
     gone = shelf["inside"][0]