fix: harden k8s security context and use SHA-tagged images

Address pr_reviewer findings:
- Enforce non-root (UID/GID 1000), no privilege escalation, drop all caps
  in the deployment securityContext.
- Tag images with the git SHA in addition to latest, and pin the deployed
  image to the SHA for idempotent rollbacks.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.gitea/workflows/build_push.yml

@@ -43,6 +43,7 @@ jobs:
           platforms: linux/amd64,linux/arm64
           tags: |
             git.aridgwayweb.com/armistace/steward:latest
+            git.aridgwayweb.com/armistace/steward:${{ gitea.sha }}
 
       - name: Deploy
         run: |
@@ -75,3 +76,4 @@ jobs:
             --from-literal=STEWARD__MATRIX__ALLOWED_USER_IDS=${{ vars.MATRIX_ALLOWED_USER_IDS }} \
             --namespace=steward
           kubectl apply -f kube/steward_deployment.yaml && kubectl apply -f kube/steward_service.yaml
+          kubectl set image deployment/steward-deployment steward=git.aridgwayweb.com/armistace/steward:${{ gitea.sha }} --namespace=steward