Fix ExternalSecret template: remove webui-secret-key reference

The open-webui item doesn't exist in 1Password yet.
webui-secret-key is manually managed via Kubernetes secret patch.
Template now only pulls vllm-api-key from 1Password.

cluster/applications/open-webui/externalsecret.yaml

@@ -18,13 +18,6 @@ spec:
       engineVersion: v2
       data:
         vllm-api-key: "{{ .vllm_api_key }}"
-        # NOTE: webui-secret-key is manually managed as a Kubernetes secret patch
-        # until the 'open-webui' item exists in 1Password with a 'secret-key' field.
-        # To re-enable automatic sync:
-        #   1. Add open-webui item to 1Password mk-labs vault
-        #   2. Add secret-key field with a strong random value
-        #   3. Uncomment the data section below and re-apply this manifest
-        webui-secret-key: "{{ .webui_secret_key | default \"PLACEHOLDER\" }}"
   data:
     # vLLM API key from 1Password (mk-labs vault, vllm item, api-key field)
     - secretKey: vllm_api_key