fix: add pod-level securityContext to secure backend deployment (CKV_K8S_29)

[Copilot]

Checkov alert CKV_K8S_29 requires both pod-level and container-level securityContext to be defined. The secure backend deployment had only a container-level context, leaving pod-wide root-execution prevention unenforced.

Change

• k8s/secure/backend/deployment.yaml — added securityContext.runAsNonRoot: true at spec.template.spec level:

    spec:
      serviceAccountName: webapp
+     securityContext:
+       runAsNonRoot: true
      containers: