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/minio/init-job.yaml

@@ -0,0 +1,24 @@
+apiVersion: batch/v1
+kind: Job
+metadata:
+  name: minio-init
+  namespace: reactbin
+spec:
+  template:
+    spec:
+      restartPolicy: OnFailure
+      containers:
+        - name: mc
+          image: minio/mc:latest
+          # mc runs as root by default; FR-013 exception documented in spec
+          securityContext:
+            runAsNonRoot: false
+          command:
+            - sh
+            - -c
+            - |
+              mc alias set local http://minio:9000 "$MINIO_ROOT_USER" "$MINIO_ROOT_PASSWORD"
+              mc mb --ignore-existing local/reactbin
+          envFrom:
+            - secretRef:
+                name: minio-credentials