Add Kubernetes/K3s deployment manifests and documentation

- Complete k8s manifests with Kustomize support
- Production and staging overlays
- ConfigMap/Secret management
- Ingress with TLS (Traefik/NGINX)
- Persistent storage for SQLite
- Comprehensive k8s README with operations guide
- Updated main README with k8s deployment instructions
- Gitignore for k8s secrets

Usage:
  kubectl apply -k k8s/overlays/production

k8s/overlays/production/namespace.yaml

@@ -0,0 +1,7 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: dyn-ddns
+  labels:
+    app.kubernetes.io/name: dyn-ddns
+    app.kubernetes.io/part-of: dws-dns