Add RBAC for backend to manage pods + update backend

- Create ServiceAccount backend-sa
- Create ClusterRole for pod management
- Bind role to ServiceAccount
- Update deployment to use ServiceAccount
- Update backend submodule with TLS fix

Co-Authored-By: Claude Sonnet 4.5 (1M context) <noreply@anthropic.com>

k8s/backend/deployment.yaml

@@ -15,6 +15,7 @@ spec:
       labels:
         app: backend
     spec:
+      serviceAccountName: backend-sa
       imagePullSecrets:
       - name: gitea-registry
       containers: