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/deployment-patch.yaml

@@ -0,0 +1,16 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: dyn-ddns
+spec:
+  template:
+    spec:
+      containers:
+      - name: dyn-ddns
+        resources:
+          requests:
+            memory: "128Mi"
+            cpu: "200m"
+          limits:
+            memory: "512Mi"
+            cpu: "1000m"