feat(stalwart): use Valkey for in-memory store, improve health probes

- Replace PostgreSQL in-memory store with Valkey (Redis-compatible) for
  better performance on rate limiting, distributed locks, and OAuth codes
- Add single-replica Valkey deployment with no persistence (data is transient)
- Switch liveness/readiness probes to HTTP GET /healthz/live and
  /healthz/ready on port 8080 per official Kubernetes probe documentation
- Update webadmin resource URL to use artifactapi proxy instead of direct
  GitHub download

apps/base/stalwart/kustomization.yaml

@@ -12,6 +12,7 @@ resources:
   - services.yaml
   - stalwart-deployment.yaml
   - stalwart-hpa.yaml
+  - valkey.yaml
   - vaultauth.yaml
   - vaultstaticsecret.yaml