fix(music-lab): cache-buster query 제거 + DALL·E prompt에 background_keyword 활용

1. video.py _container_to_nas, orchestrator.py _local_path에서 path 변환 전 ?쿼리 strip
   — 이전 commit 20c5268의 cache-buster ?v=...가 Windows path로 그대로 전달되어 input_validation 실패하던 문제 픽스
2. cover.py _generate_with_dalle가 background_keyword를 prompt에 포함
   — 사용자가 PipelineStartModal에서 '배경 키워드' 입력 시 처음부터 원하는 분위기 cover 생성

music-lab/tests/test_cover_generation.py

@@ -147,3 +147,30 @@ async def test_pexels_zero_results_falls_back(tmp_storage, monkeypatch):
         image_source="pexels",
     )
     assert out["used_fallback"] is True
+
+
+@pytest.mark.asyncio
+@respx.mock
+async def test_dalle_uses_background_keyword(tmp_storage, monkeypatch):
+    monkeypatch.setenv("OPENAI_API_KEY", "test-key")
+    captured = {}
+    def hook(req):
+        import json as _json
+        captured["body"] = _json.loads(req.content)
+        return Response(200, json={"data": [{"url": "https://x"}]})
+    respx.post("https://api.openai.com/v1/images/generations").mock(side_effect=hook)
+    png_bytes = bytes.fromhex(
+        "89504e470d0a1a0a0000000d49484452000000010000000108020000009077"
+        "53de0000000c4944415478da6300010000050001"
+        "0d0a2db40000000049454e44ae426082"
+    )
+    respx.get("https://x").mock(return_value=Response(200, content=png_bytes))
+    await cover.generate(
+        pipeline_id=80, genre="lo-fi",
+        prompt_template="moody anime",
+        mood="chill", track_title="X",
+        image_source="ai",
+        background_keyword="skateboard park bright atmosphere",
+    )
+    assert "skateboard" in captured["body"]["prompt"]
+    assert "bright" in captured["body"]["prompt"]