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_api_tokens.py

@@ -25,16 +25,8 @@ from app.config import settings
 from app.db import SessionLocal, get_db
 from app.main import app
 from app.notifications import create_notification
-from app.models import (
-    ApiToken,
-    ApiTokenGrant,
-    AuditLog,
-    DownloadJob,
-    FileFolder,
-    MediaAsset,
-    Notification,
-    User,
-)
+from app.models import ApiToken, ApiTokenGrant, AuditLog, Notification, User
+from app.downloads.models import DownloadJob, FileFolder, MediaAsset
 from app.ratelimit import RateLimiter
 from app.routes import tokens as tokens_routes
 from app.routes.auth import bump_session_epoch