fix(music-lab): retry 레이스 가드(retrying 전이) + failed_step 검증 + backoff 빈리스트 가드

- Fix 1: retry_pipeline이 bg.add_task 직전 상태를 'retrying'으로 전이 → 동시 retry 409 방지
- Fix 2: test_retry_failed_pipeline_retriggers에 called[pid/step] assert 추가
- Fix 3: failed_step이 STEPS에 없으면 409 (엉뚱한 prefix 방지)
- Fix 4: STEP_RETRY_BACKOFF_SEC 빈 리스트 시 IndexError → 0으로 폴백

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

music-lab/tests/test_pipeline_retry.py

@@ -122,3 +122,53 @@ def test_retry_publish_with_video_id_rejected(fresh_db, client):
                              youtube_video_id="abc123")
     r = client.post(f"/api/music/pipeline/{pid}/retry")
     assert r.status_code == 409
+
+
+# ── Fix 2: fake_run 인자 검증 ────────────────────────────────────────────────
+
+def test_retry_failed_pipeline_retriggers_with_correct_args(fresh_db, client, monkeypatch):
+    """fake_run이 (pid, failed_step)으로 호출되는지 검증."""
+    pid = db.create_pipeline(track_id=1)
+    job = db.create_pipeline_job(pid, "video")
+    db.update_pipeline_job(job, status="failed", error="boom")
+    db.update_pipeline_state(pid, "failed", failed_reason="video: boom")
+    called = {}
+
+    async def fake_run(p, step, *a):
+        called["pid"], called["step"] = p, step
+
+    monkeypatch.setattr(orchestrator, "run_step", fake_run)
+    r = client.post(f"/api/music/pipeline/{pid}/retry")
+    assert r.status_code in (200, 202)
+    assert called["pid"] == pid
+    assert called["step"] == "video"
+
+
+# ── Fix 1: retrying 전이로 중복 retry 409 ────────────────────────────────────
+
+def test_retry_twice_second_is_409(fresh_db, client, monkeypatch):
+    """첫 번째 retry가 상태를 'retrying'으로 전이 → 두 번째 retry는 409."""
+    pid = db.create_pipeline(track_id=1)
+    job = db.create_pipeline_job(pid, "video")
+    db.update_pipeline_job(job, status="failed", error="boom")
+    db.update_pipeline_state(pid, "failed", failed_reason="video: boom")
+
+    async def fake_run(p, step, *a):
+        pass
+
+    monkeypatch.setattr(orchestrator, "run_step", fake_run)
+    r1 = client.post(f"/api/music/pipeline/{pid}/retry")
+    assert r1.status_code in (200, 202)
+    r2 = client.post(f"/api/music/pipeline/{pid}/retry")  # 이미 retrying → 409
+    assert r2.status_code == 409
+
+
+# ── Fix 3: 알 수 없는 step prefix → 409 ─────────────────────────────────────
+
+def test_retry_unparseable_failed_reason_409(fresh_db, client):
+    """failed_reason이 known STEPS에 없는 prefix면 409."""
+    pid = db.create_pipeline(track_id=1)
+    # failed job row 없이 state만 failed + 비-step prefix reason
+    db.update_pipeline_state(pid, "failed", failed_reason="ValueError: track 1 없음")
+    r = client.post(f"/api/music/pipeline/{pid}/retry")
+    assert r.status_code == 409