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/stalwart-deployment.yaml

@@ -66,16 +66,18 @@ spec:
                   name: stalwart-admin
                   key: password_hash
           livenessProbe:
-            tcpSocket:
-              port: 25
+            httpGet:
+              path: /healthz/live
+              port: 8080
             initialDelaySeconds: 30
-            periodSeconds: 30
+            periodSeconds: 10
             timeoutSeconds: 5
             failureThreshold: 3
           readinessProbe:
-            tcpSocket:
-              port: 25
-            initialDelaySeconds: 15
+            httpGet:
+              path: /healthz/ready
+              port: 8080
+            initialDelaySeconds: 5
             periodSeconds: 10
             timeoutSeconds: 3
             failureThreshold: 3