Feat: Add Kubernetes manifests for k3s production deployment

Adds complete k8s/ manifest tree: Namespace, VaultAuth + VaultStaticSecret
CRDs (VSO secret sync from Vault KV v2), API and UI Deployments and Services,
nginx Ingress with cert-manager TLS, MinIO StatefulSet with PVC and init Job,
and Alembic init container on the API Deployment for automatic schema
migrations. Includes .yamllint.yml config and validate-k8s Makefile target.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

k8s/ui/deployment.yaml

@@ -0,0 +1,30 @@
+# Replace 'latest' with the real image tag before applying
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: ui
+  namespace: reactbin
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: ui
+  template:
+    metadata:
+      labels:
+        app: ui
+    spec:
+      containers:
+        - name: ui
+          image: reactbin-ui:latest
+          ports:
+            - containerPort: 8080
+          livenessProbe:
+            httpGet:
+              path: /
+              port: 8080
+            initialDelaySeconds: 10
+            periodSeconds: 30
+          securityContext:
+            runAsNonRoot: true
+            runAsUser: 101  # nginxinc/nginx-unprivileged default UID

k8s/ui/service.yaml

@@ -0,0 +1,13 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: ui
+  namespace: reactbin
+spec:
+  type: ClusterIP
+  selector:
+    app: ui
+  ports:
+    - name: http
+      port: 8080
+      targetPort: 8080