feat(music-lab): AI 커버 생성 + 그라데이션 폴백

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

music-lab/app/pipeline/storage.py

@@ -0,0 +1,15 @@
+"""파이프라인 산출물 디렉토리 관리."""
+import os
+
+VIDEO_DATA_DIR = os.getenv("VIDEO_DATA_DIR", "/app/data/videos")
+VIDEO_MEDIA_BASE = os.getenv("VIDEO_MEDIA_BASE", "/media/videos")
+
+
+def pipeline_dir(pipeline_id: int) -> str:
+    path = os.path.join(VIDEO_DATA_DIR, str(pipeline_id))
+    os.makedirs(path, exist_ok=True)
+    return path
+
+
+def media_url(pipeline_id: int, filename: str) -> str:
+    return f"{VIDEO_MEDIA_BASE}/{pipeline_id}/{filename}"