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/kustomization.yaml

@@ -0,0 +1,28 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - namespace.yaml
+  - ../../base
+  - secrets.yaml
+
+namePrefix: prod-
+namespace: dyn-ddns
+
+commonLabels:
+  environment: production
+
+configMapGenerator:
+  - name: dyn-ddns-config
+    behavior: merge
+    literals:
+      - TECHNITIUM_URL=https://dns.dws.rip
+      - RATE_LIMIT_PER_IP=10
+      - RATE_LIMIT_PER_TOKEN=1
+
+patchesStrategicMerge:
+  - deployment-patch.yaml
+
+replicas:
+  - name: dyn-ddns
+    count: 2