fix(music-render): drop secondary webhook_add_track (T5 follow-up)

Code review found: f"{task_id}_v2" / "_inst" synthetic task IDs never
exist in NAS music_tasks table -> webhook returns 404 -> silent fail.

NAS music-lab/main.py._sync_library_with_disk() auto-registers any
.mp3 in the disk that has no DB row on next GET /api/music/library.
So Windows worker just writes the file to SMB; NAS picks it up on
the next library fetch -- matches NAS source behavior at file level.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

services/music-render/providers/suno.py

@@ -250,11 +250,9 @@ def run_suno_generation(task_id: str, params: dict) -> None:
 
         if len(completed) > 1:
             try:
-                second = _download_and_register(f"{task_id}_v2", completed[1], params)
-                if second:
-                    # 두 번째 변형은 별도 task가 아니라 별도 track으로만 등록
-                    webhook_add_track(f"{task_id}_v2", "succeeded", 100, "두 번째 변형",
-                                      audio_url=second["audio_url"], track=second)
+                # 보조 변형은 SMB에 파일만 저장. NAS _sync_library_with_disk가 다음
+                # GET /api/music/library 호출 시 자동으로 라이브러리에 등록.
+                _download_and_register(f"{task_id}_v2", completed[1], params)
             except Exception:
                 pass
 
@@ -348,10 +346,8 @@ def run_vocal_removal(task_id: str, params: dict) -> None:
         track = _download_and_register(task_id, completed[0], vp)
         if len(completed) > 1:
             ip = {**params, "title": f"{params.get('title', 'Track')} (Instrumental)"}
-            second = _download_and_register(f"{task_id}_inst", completed[1], ip)
-            if second:
-                webhook_add_track(f"{task_id}_inst", "succeeded", 100, "Instrumental",
-                                  audio_url=second["audio_url"], track=second)
+            # Instrumental 변형은 SMB에 파일만 저장. NAS _sync_library_with_disk가 자동 등록.
+            _download_and_register(f"{task_id}_inst", completed[1], ip)
         if track:
             webhook_add_track(task_id, "succeeded", 100, "보컬 분리 완료",
                               audio_url=track["audio_url"], track=track)