When NVIDIA_NIM_API_KEY or OPENROUTER_API_KEY is empty or not set,
the proxy forwarded requests without a valid Authorization header,
causing providers to return 403 with 'Header of type authorization
was missing'.
Now fail fast with HTTP 503 and a clear message telling users to add
the key to .env, with links to obtain keys.
Fixes#29
Co-authored-by: Ali Khokhar <alishahryar2@gmail.com>