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/secrets.yaml

@@ -0,0 +1,17 @@
+apiVersion: v1
+kind: Secret
+metadata:
+  name: dyn-ddns-secrets
+type: Opaque
+stringData:
+  # Choose ONE authentication method:
+  
+  # Method 1: API Token (recommended)
+  # TECHNITIUM_TOKEN: "your-api-token-here"
+  
+  # Method 2: Username/Password
+  # TECHNITIUM_USERNAME: "admin"
+  # TECHNITIUM_PASSWORD: "your-password"
+  
+  # Optional: Trusted proxies (comma-separated)
+  # TRUSTED_PROXIES: "10.0.0.0/8,172.16.0.0/12"