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/valkey.yaml

@@ -0,0 +1,58 @@
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: stalwart-valkey
+  namespace: stalwart
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: stalwart-valkey
+  template:
+    metadata:
+      labels:
+        app: stalwart-valkey
+    spec:
+      containers:
+        - name: valkey
+          image: valkey/valkey:8-alpine
+          args:
+            - "--save"
+            - ""
+            - "--appendonly"
+            - "no"
+          ports:
+            - containerPort: 6379
+              name: valkey
+              protocol: TCP
+          resources:
+            requests:
+              cpu: 50m
+              memory: 64Mi
+            limits:
+              cpu: 500m
+              memory: 256Mi
+          livenessProbe:
+            tcpSocket:
+              port: 6379
+            initialDelaySeconds: 5
+            periodSeconds: 10
+          readinessProbe:
+            exec:
+              command: ["valkey-cli", "ping"]
+            initialDelaySeconds: 5
+            periodSeconds: 5
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: stalwart-valkey
+  namespace: stalwart
+spec:
+  selector:
+    app: stalwart-valkey
+  ports:
+    - port: 6379
+      targetPort: 6379
+      name: valkey