Save the user's ElevenLabs API key

Save the USER'S OWN ElevenLabs API key so this machine can generate real voiceovers. Call this only when a tool reports that no key is configured, or when the user offers a key. Ask the user for their key first — get one free at https://elevenlabs.io/app/settings/api-keys — and NEVER invent one. The key is written to ~/.config/video-studio/config.json with 0600 permissions and never leaves this machine. Alternative for users who prefer not to type a key into chat: put ELEVENLABS_API_KEY in the 'env' block of their MCP client config instead; that always wins over the saved file. Call with no arguments to just report where the current key comes from.