Generate a song from a prompt with optional custom lyrics and style inputs. Returns JSON describing the generated track

Generate a song from a prompt with optional custom lyrics and style inputs. Returns JSON describing the generated track when the request succeeds. PAYMENT-SIGNATURE-authenticated requests default to asynchronous execution. Asynchronous responses return 202 Accepted with a songId and pollUrl. Poll GET /api/songs/{songId} without another payment until model_version leaves 'pending' ('failed' is terminal); audio_url is a placeholder image until then and the finished MP3 after.