feat(music-lab): POST /pipeline/{id}/retry — 실패 step 수동 재개

terminal failed 파이프라인을 마지막 실패 step부터 재개.
publish + youtube_video_id 있으면 중복 업로드 방지 409.
pytest.ini에 pythonpath=.. 추가 (PYTHONPATH=.. 없이 TestClient 테스트 구동).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

music-lab/app/main.py

@@ -1128,6 +1128,25 @@ def cancel_pipeline(pid: int):
     return {"ok": True}
 
 
+@app.post("/api/music/pipeline/{pid}/retry", status_code=202)
+async def retry_pipeline(pid: int, bg: BackgroundTasks):
+    p = _db_module.get_pipeline(pid)
+    if not p:
+        raise HTTPException(404)
+    if p["state"] != "failed":
+        raise HTTPException(409, f"재개 불가 (state={p['state']})")
+    failed_step = _db_module.get_last_failed_step(pid)
+    if not failed_step:
+        reason = p.get("failed_reason") or ""
+        failed_step = reason.split(":", 1)[0].strip() or None
+    if not failed_step:
+        raise HTTPException(409, "실패 step을 판별할 수 없음")
+    if failed_step == "publish" and p.get("youtube_video_id"):
+        raise HTTPException(409, "이미 업로드됨 (중복 방지)")
+    bg.add_task(orchestrator.run_step, pid, failed_step)
+    return {"ok": True, "retrying_step": failed_step}
+
+
 @app.post("/api/music/pipeline/{pid}/publish", status_code=202)
 async def publish_pipeline(pid: int, bg: BackgroundTasks):
     p = _db_module.get_pipeline(pid)