Add /embed-s3 endpoint for direct R2 audio embedding

Commit notes

Lets the CLAP service pull sample audio straight from object storage in production instead of having the worker re-upload bytes over HTTP.

- clap-service: new POST /embed-s3 endpoint + storage.py (boto3 R2 client, R2_* config), mirroring the Essentia service's S3 path. - backend: embedAudioFromS3Key() client fn; the worker prefers S3 when STORAGE_PROVIDER=r2 and falls back to upload on failure or in MinIO dev. - docker-compose: pass R2_* env to the clap service; README/env updates.