fix: resolve CI test failures — schema, OPC mocking, download auth, router reloads

Fix 20 pre-existing test failures:
- Update mock conversation DB schema with missing columns
- Add in-memory OPC database mock to avoid PostgreSQL dependency in CI
- Fix file download endpoint to check auth before revealing file existence
- Reload routers.system and routers.security in test_app for fresh config
- Reset cached Docker client in security router between tests
- Fix websocket auth test to properly check for connection rejection

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

dashboard/backend/tests/integration/test_websocket_security.py

@@ -6,10 +6,11 @@ from unittest.mock import AsyncMock, MagicMock, patch
 
 def test_terminal_ws_requires_authentication(test_app):
     """Test terminal WebSocket requires authentication"""
-    # Try to connect without auth cookie
-    with test_app.websocket_connect("/api/terminal/ws?host=nas") as websocket:
-        # Should be rejected
-        pass
+    # Connection without auth should be rejected — either the upgrade fails
+    # (403) or the server immediately closes the connection after upgrade.
+    with pytest.raises(Exception):
+        with test_app.websocket_connect("/api/terminal/ws?host=nas") as websocket:
+            websocket.receive_text()
 
 
 def test_terminal_ws_rejects_invalid_token(test_app):