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/files.py

@@ -44,16 +44,14 @@ from app.downloads import folders, preview, quota, service, storage
 from app.downloads import upload as uploadmod
 from app.downloads.status import JobStatus
 from app.mailtext import lang_for_user
-from app.models import (
-    DISPLAY_TEXT_MAX,
+from app.models import DISPLAY_TEXT_MAX, User, display_text
+from app.downloads.models import (
     DownloadJob,
     DownloadLink,
     DownloadShare,
     FileFolder,
     LibraryState,
     MediaAsset,
-    User,
-    display_text,
 )
 from app.refusal import refused
 from app.routes._common import capped_stream_to, not_found, owned_or_404