fix(music-lab): xfade offset 누적 오차 수정 + 테스트 보강

- _build_slideshow_cmd: offset 공식을 `duration_per_image * i - xd * i`로 수정 (누적 전환 오차 제거)
- _generate_metadata: genre 빈 문자열일 때 yt_tags에 빈 문자열 삽입 방지
- test: VIDEO_DATA_DIR 패치를 monkeypatch로 교체 (자동 복원 보장)
- test: xfade offset 값 검증 테스트 추가 (29.00, 58.00)
- test: 미사용 import 제거 (pytest, sqlite3)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

music-lab/tests/test_video_producer.py

@@ -1,7 +1,6 @@
 # music-lab/tests/test_video_producer.py
 import os
 from unittest.mock import patch, MagicMock
-import pytest
 
 
 def test_build_visualizer_cmd():
@@ -59,10 +58,9 @@ def test_build_slideshow_cmd_multiple_images():
     assert "/tmp/out.mp4" in cmd
 
 
-def test_produce_video_visualizer_calls_ffmpeg(tmp_db, tmp_path):
+def test_produce_video_visualizer_calls_ffmpeg(tmp_db, tmp_path, monkeypatch):
     """produce_video가 visualizer 포맷으로 FFmpeg를 호출하는지 확인."""
     from app.db import init_db, create_video_project
-    import sqlite3
 
     init_db()
 
@@ -82,7 +80,7 @@ def test_produce_video_visualizer_calls_ffmpeg(tmp_db, tmp_path):
     create_video_project({"track_id": 1, "format": "visualizer", "target_countries": ["BR"]})
 
     import app.video_producer as vp
-    vp.VIDEO_DATA_DIR = str(tmp_path / "videos")
+    monkeypatch.setattr("app.video_producer.VIDEO_DATA_DIR", str(tmp_path / "videos"))
 
     with patch("app.video_producer.subprocess.run") as mock_run, \
          patch("app.video_producer._generate_metadata", return_value={
@@ -96,3 +94,18 @@ def test_produce_video_visualizer_calls_ffmpeg(tmp_db, tmp_path):
     proj = get_video_project(1)
     assert proj["status"] == "done"
     assert mock_run.called
+
+
+def test_build_slideshow_cmd_offset_calculation():
+    from app.video_producer import _build_slideshow_cmd
+    imgs = ["/tmp/img0.jpg", "/tmp/img1.jpg", "/tmp/img2.jpg"]
+    cmd = _build_slideshow_cmd(imgs, "/tmp/audio.mp3", "/tmp/out.mp4", duration_per_image=30.0)
+    # filter_complex 문자열 추출
+    fc_idx = cmd.index("-filter_complex")
+    fc = cmd[fc_idx + 1]
+    # xfade이 2번 등장해야 함 (이미지 3개 → 전환 2번)
+    assert fc.count("xfade") == 2
+    # 첫 번째 xfade offset: 30*1 - 1*1 = 29.0
+    assert "offset=29.00" in fc
+    # 두 번째 xfade offset: 30*2 - 1*2 = 58.0
+    assert "offset=58.00" in fc