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/vault/minio-secret.yaml

@@ -0,0 +1,16 @@
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultStaticSecret
+metadata:
+  name: minio-secret
+  namespace: reactbin
+spec:
+  vaultAuthRef: reactbin-auth
+  mount: secret
+  type: kv-v2
+  # Required Vault keys at this path:
+  #   MINIO_ROOT_USER, MINIO_ROOT_PASSWORD
+  path: reactbin/minio/credentials
+  refreshAfter: 1h
+  destination:
+    name: minio-credentials
+    create: true