Create a task

Submit a music, lyrics, upload, or concat task. Returns immediately with a task_id; poll GET /api/v1/task/{taskId} for completion. Credits are frozen at submission and either settled on success or refunded on failure. Input is validated before any credits, task record, or upstream-provider side effect. Task creation does not currently support idempotency keys; treat an uncertain network or 5xx outcome as unknown rather than automatically submitting the same payload again.