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/vault-auth.yaml

@@ -0,0 +1,16 @@
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultAuth
+metadata:
+  name: reactbin-auth
+  namespace: reactbin
+spec:
+  method: kubernetes
+  mount: kubernetes
+  kubernetes:
+    # The operator must create this role in Vault and bind it to the
+    # default service account in the reactbin namespace with read access
+    # to both reactbin/api/config and reactbin/minio/credentials.
+    role: reactbin
+    serviceAccount: default
+    audiences:
+      - https://kubernetes.default.svc