- New helpers: parse_stems_urls, upload_to_gokapi, process_stems_to_gokapi
- Stems webhook handler now downloads stem files from MusicGPT CDN,
zips them, uploads to Gokapi with 30-day expiry, saves the Gokapi
download URL to stems_link in the DB
- Download Stems button uses stems_link (Gokapi URL) when available,
falls back to the zip endpoint if not
- Config: GOKAPI_URL, GOKAPI_API_KEY, GOKAPI_EXPIRY_DAYS env vars
- docker-compose: GOKAPI_* environment variables added