Add ElevenLabs API Key Support to Settings Model

Add support for storing ElevenLabs API keys in the existing Settings system to enable users to configure their ElevenLabs text-to-speech integration.

Tasks: 
- Add `elevenLabs: {}` field to Settings schema in `api/models/Settings.js`
- Verify existing `updateSettings()` and `getSettings()` endpoints work with new field
- Test Settings API accepts nested `elevenLabs.apiKey` structure