fix(music-render): F6 ReliableQueue 적용 (F6 part 3)

- worker.py: poll_once 신설, BLPOP → ReliableQueue.dequeue/ack/fail + startup recovery
- 12 job_type dispatch table 보존 (기존 13 tests 그대로 PASS)
- Dockerfile: build context=services/, _shared 포함, PYTHONPATH=/app
- docker-compose.yml: music-render build context 갱신

dispatch 자체 unhandled exception 발생 시 fail(raw, payload)로 retry/dead-letter.
provider 함수가 webhook("failed")를 잡고 있는 정상 케이스는 ack (멱등 webhook).

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

services/music-render/tests/test_worker.py

@@ -107,3 +107,63 @@ def test_dispatch_add_instrumental_calls_run_add_instrumental():
     with patch("worker.run_add_instrumental") as m:
         worker._dispatch(payload)
         m.assert_called_once_with("t13", {"upload_url": "u"})
+
+
+# ----- F6: ReliableQueue poll_once -----
+
+from unittest.mock import AsyncMock
+
+
+@pytest.mark.asyncio
+async def test_poll_once_acks_on_success(monkeypatch):
+    """F6 — _dispatch 정상 return → queue.ack(raw)."""
+    payload = {"task_id": "t1", "job_type": "suno_generation", "params": {}}
+    raw = json.dumps(payload).encode()
+    fake_queue = AsyncMock()
+    fake_queue.dequeue = AsyncMock(return_value=(payload, raw))
+    fake_queue.ack = AsyncMock()
+    fake_queue.fail = AsyncMock()
+
+    monkeypatch.setattr(worker, "_dispatch", MagicMock())
+
+    handled = await worker.poll_once(fake_queue)
+    assert handled is True
+    fake_queue.ack.assert_awaited_once_with(raw)
+    fake_queue.fail.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_poll_once_calls_fail_on_dispatch_exception(monkeypatch):
+    """F6 — _dispatch unhandled exception → queue.fail(raw, payload)."""
+    payload = {"task_id": "t2", "job_type": "suno_generation", "params": {}}
+    raw = json.dumps(payload).encode()
+    fake_queue = AsyncMock()
+    fake_queue.dequeue = AsyncMock(return_value=(payload, raw))
+    fake_queue.ack = AsyncMock()
+    fake_queue.fail = AsyncMock()
+
+    def _boom(p):
+        raise RuntimeError("dispatch crash")
+
+    monkeypatch.setattr(worker, "_dispatch", _boom)
+
+    handled = await worker.poll_once(fake_queue)
+    assert handled is True
+    fake_queue.fail.assert_awaited_once_with(raw, payload)
+    fake_queue.ack.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_poll_once_returns_false_on_timeout(monkeypatch):
+    fake_queue = AsyncMock()
+    fake_queue.dequeue = AsyncMock(return_value=None)
+    fake_queue.ack = AsyncMock()
+    fake_queue.fail = AsyncMock()
+    dispatch_mock = MagicMock()
+    monkeypatch.setattr(worker, "_dispatch", dispatch_mock)
+
+    handled = await worker.poll_once(fake_queue)
+    assert handled is False
+    dispatch_mock.assert_not_called()
+    fake_queue.ack.assert_not_awaited()
+    fake_queue.fail.assert_not_awaited()