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/base/configmap.yaml

@@ -0,0 +1,13 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: dyn-ddns-config
+data:
+  SERVER_PORT: "8080"
+  DATABASE_PATH: "/data/dyn.db"
+  BASE_DOMAIN: "dws.rip"
+  SPACE_SUBDOMAIN: "space"
+  RATE_LIMIT_PER_IP: "10"
+  RATE_LIMIT_PER_TOKEN: "1"
+  # TECHNITIUM_URL: "https://dns.dws.rip"
+  # Add your Technitium URL above or in overlays