cleanup: remove MongoDB dependencies and manifests

- Delete MongoDB StatefulSet and service from Kubernetes
- Remove mongodb-statefulset.yaml manifest file
- Remove mongodb-memory-server from devDependencies
- MongoDB no longer needed after CouchDB migration

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backend/package.json

@@ -39,7 +39,7 @@
     "eslint": "^9.38.0",
     "jest": "^30.2.0",
     "jest-environment-node": "^30.2.0",
-    "mongodb-memory-server": "^10.3.0",
+    
     "socket.io-client": "^4.8.1",
     "supertest": "^7.1.4"
   }