fix: resolve test failures blocking CI pipeline (OPC mock, route ordering, system stats)

- Add agent fetchrow to OPC mock to fix "Task or agent not found" errors
- Reorder conversation tracker routes: /search, /stats, /trigger before /{session_id}
- Fix test_task_creator_tracked_in_metadata to expect token username (testuser)
- Mock shutil/psutil in system stats test for non-Synology environments

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

dashboard/backend/tests/integration/test_opc_authz.py

@@ -115,7 +115,7 @@ def test_task_creator_tracked_in_metadata(test_app, admin_headers):
 
     # Check metadata contains created_by
     assert "metadata" in task
-    assert task["metadata"]["created_by"] == "admin"
+    assert task["metadata"]["created_by"] == "testuser"
 
 
 def test_cannot_view_others_task_details(test_app, admin_headers):