Initialize skin analysis progress tracking

POST https://api.sandbox.oysterskin.com/api/v1/skin/analyze-with-progress/init

Creates a backend-issued progress tracking session for the progress-enabled skin analysis endpoint.
The response includes a one-time tracking_id and tracking_token.

Recommended client flow

1. Call this endpoint
2. Subscribe to channel analysis-progress.{tracking_id}
3. Call /skin/analyze-with-progress with the returned tracking credentials