Auto-upload stems to Gokapi on completion, save share link

- 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

config.py

@@ -110,6 +110,11 @@ class Config:
     MUSICGPT_MODELS = ['v6', 'v6-pro']
     MUSICGPT_DEFAULT_MODEL = os.environ.get('MUSICGPT_DEFAULT_MODEL', 'v6-pro')
 
+    # Gokapi file-sharing server for stems uploads.
+    GOKAPI_URL = os.environ.get('GOKAPI_URL', '')
+    GOKAPI_API_KEY = os.environ.get('GOKAPI_API_KEY', '')
+    GOKAPI_EXPIRY_DAYS = int(os.environ.get('GOKAPI_EXPIRY_DAYS', '30'))
+
     @classmethod
     def musicgpt_webhook_base_url(cls):
         """Return the public base URL used for MusicGPT webhooks."""