feat(music-lab): verify_internal_key + /api/internal/music/update (SP-6)

X-Internal-Key 헤더 검증 dependency (insta-lab 동일 패턴).
Windows music-render webhook 수신 endpoint — update_task + 옵션 add_track.
Plan-B-Music Phase 1 (수신부).

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

music-lab/tests/test_auth.py

@@ -0,0 +1,23 @@
+"""verify_internal_key dependency — Windows music-render webhook 인증."""
+import pytest
+from fastapi import HTTPException
+from app.auth import verify_internal_key
+
+
+def test_valid_key_passes(monkeypatch):
+    monkeypatch.setenv("INTERNAL_API_KEY", "secret123")
+    verify_internal_key(x_internal_key="secret123")
+
+
+def test_invalid_key_raises_401(monkeypatch):
+    monkeypatch.setenv("INTERNAL_API_KEY", "secret123")
+    with pytest.raises(HTTPException) as exc:
+        verify_internal_key(x_internal_key="wrong")
+    assert exc.value.status_code == 401
+
+
+def test_missing_env_key_raises_401(monkeypatch):
+    monkeypatch.delenv("INTERNAL_API_KEY", raising=False)
+    with pytest.raises(HTTPException) as exc:
+        verify_internal_key(x_internal_key="any")
+    assert exc.value.status_code == 401