feat(postfix): deploy postfix MTA and rspamd spam filter

- mailgateway namespace with Deployment + HPA (2-6 replicas)
- rspamd Deployment + HPA (2-6 replicas) with milter interface
- postfix configured to relay inbound mail to stalwart via transport maps
- rspamd milter on port 11332 for spam scanning and DKIM signing
- DKIM keys stored in Vault at kubernetes/namespace/mailgateway/default/dkim-keys
- TLS cert via cert-manager (vault-issuer) for mail.main.unkin.net
- rspamd web UI exposed via Traefik Gateway at rspamd.k8s.syd1.au.unkin.net
- postfix external LoadBalancer service for inbound MX on port 25
- Add full main.cf and master.cf as ConfigMap resources mounted via subPath
  - main.cf: relay-only gateway config, texthash: transport maps, rspamd milter
  - master.cf: standard smtp + submission (587, TLS required) + internal processes
- MAILNAME/MY_NETWORKS/MY_DESTINATION env vars kept in sync with main.cf
- LOG_TO_STDOUT=1 for k8s log collection

apps/base/mailgateway/rspamd-deployment.yaml

@@ -0,0 +1,75 @@
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: rspamd
+  namespace: mailgateway
+spec:
+  selector:
+    matchLabels:
+      app: rspamd
+  template:
+    metadata:
+      annotations:
+        reloader.stakater.com/auto: "true"
+      labels:
+        app: rspamd
+    spec:
+      securityContext:
+        runAsUser: 11333
+        runAsGroup: 11333
+        fsGroup: 11333
+      containers:
+        - name: rspamd
+          image: rspamd/rspamd:4.0.1
+          ports:
+            - containerPort: 11332
+              name: milter
+              protocol: TCP
+            - containerPort: 11333
+              name: worker
+              protocol: TCP
+            - containerPort: 11334
+              name: controller
+              protocol: TCP
+          livenessProbe:
+            httpGet:
+              path: /ping
+              port: 11334
+            initialDelaySeconds: 15
+            periodSeconds: 30
+            timeoutSeconds: 5
+            failureThreshold: 3
+          readinessProbe:
+            httpGet:
+              path: /ping
+              port: 11334
+            initialDelaySeconds: 10
+            periodSeconds: 10
+            timeoutSeconds: 3
+            failureThreshold: 3
+          resources:
+            requests:
+              cpu: 100m
+              memory: 256Mi
+            limits:
+              cpu: "1"
+              memory: 512Mi
+          volumeMounts:
+            - name: rspamd-config
+              mountPath: /etc/rspamd/local.d
+              readOnly: true
+            - name: dkim-keys
+              mountPath: /etc/rspamd/dkim
+              readOnly: true
+            - name: rspamd-data
+              mountPath: /var/lib/rspamd
+      volumes:
+        - name: rspamd-config
+          configMap:
+            name: rspamd-config
+        - name: dkim-keys
+          secret:
+            secretName: dkim-keys
+        - name: rspamd-data
+          emptyDir: {}