feat(gotify): add push notification server + SMTP email relay

- Gotify server (ghcr.io/gotify/server:2.6.1) in notifications namespace
- SMTP emailer sidecar polls messages, forwards as email via msmtp
- Ingress at gotify.riotpiao.com with WebSocket support
- 1Gi Longhorn PVC for message persistence
- ArgoCD Application (wave 8, auto-sync)
- Secrets template for admin creds, SMTP config, tokens
- README with setup guide: tokens, Forgejo webhooks, SMTP providers

Enables PR created/merged email notifications from Forgejo.

k8s/apps/gotify/service.yaml

@@ -0,0 +1,14 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: gotify
+  namespace: notifications
+  labels:
+    app: gotify
+spec:
+  selector:
+    app: gotify
+  ports:
+    - port: 80
+      targetPort: 80
+      protocol: TCP