Add automatic API key attachment to backend client

[Jayy4rl]

Context: The backend client (Issue 382) does not attach the API key automatically. Consumers must set the header manually on every call.

Requirements:

• Add apiKey option to createBackendClient.
• When set, automatically attach Authorization: Bearer <apiKey> to all requests.

Acceptance criteria:

• Requests to admin endpoints include the Authorization header when apiKey is configured.
• Requests without apiKey send no Authorization header.