Add endpoint to generate melspectrogram

As we are already using the chirp module: https://github.com/developmentseed/google-bioacoustics-embed-service/blob/main/Dockerfile#L15, we could consider adding an endpoint to generate the melspectrogram, instead of doing it in the API service.

Start with Dockerfile line 15 to confirm how the chirp module is included, then trace where the API service currently generates the melspectrogram. Define the endpoint's expected inputs and output from the existing service behavior; done means the service exposes that generation through the new endpoint.

Written by the indexing model from the issue text.