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/api-secret.yaml

@@ -0,0 +1,18 @@
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultStaticSecret
+metadata:
+  name: api-secret
+  namespace: reactbin
+spec:
+  vaultAuthRef: reactbin-auth
+  mount: secret
+  type: kv-v2
+  # Required Vault keys at this path:
+  #   DATABASE_URL, JWT_SECRET_KEY, OWNER_USERNAME, OWNER_PASSWORD,
+  #   S3_ENDPOINT_URL, S3_BUCKET_NAME, S3_ACCESS_KEY_ID, S3_SECRET_ACCESS_KEY,
+  #   API_BASE_URL
+  path: reactbin/api/config
+  refreshAfter: 1h
+  destination:
+    name: api-env
+    create: true

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

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