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/tests/test_pipeline_retry.py

@@ -1,4 +1,5 @@
 import pytest
+from fastapi.testclient import TestClient
 from app import db
 from app.pipeline import orchestrator
 
@@ -81,3 +82,43 @@ async def test_publish_not_retried(fresh_db, monkeypatch):
     await orchestrator.run_step(pid, "publish")
     assert calls["n"] == 1
     assert db.get_pipeline(pid)["state"] == "failed"
+
+
+# ── Task 3: retry endpoint tests ─────────────────────────────────────────────
+
+@pytest.fixture
+def client(fresh_db):
+    from app.main import app
+    return TestClient(app)
+
+
+def test_retry_failed_pipeline_retriggers(fresh_db, client, monkeypatch):
+    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 r.json()["retrying_step"] == "video"
+
+
+def test_retry_non_failed_409(fresh_db, client):
+    pid = db.create_pipeline(track_id=1)  # state='created'
+    r = client.post(f"/api/music/pipeline/{pid}/retry")
+    assert r.status_code == 409
+
+
+def test_retry_publish_with_video_id_rejected(fresh_db, client):
+    pid = db.create_pipeline(track_id=1)
+    job = db.create_pipeline_job(pid, "publish")
+    db.update_pipeline_job(job, status="failed", error="x")
+    db.update_pipeline_state(pid, "failed", failed_reason="publish: x",
+                             youtube_video_id="abc123")
+    r = client.post(f"/api/music/pipeline/{pid}/retry")
+    assert r.status_code == 409