Add backup-first duplicate repair

tests/test_web.py

@@ -3,7 +3,7 @@ from pathlib import Path
 
 import pytest
 
-from serato_doctor.web import STATIC_FILES, analyze_paths
+from serato_doctor.web import STATIC_FILES, analyze_paths, duplicate_repair
 
 
 def test_web_analysis_uses_production_health_pipeline(tmp_path):
@@ -49,3 +49,24 @@ def test_web_static_assets_are_declared_and_packaged():
     assert 'data-detail="database_missing_tracks"' in html
     assert 'data-detail="old_crate_references"' in html
     assert html.count('class="info-button"') >= 10
+
+
+def test_duplicate_repair_preview_does_not_change_files(tmp_path):
+    serato = tmp_path / "_Serato_"
+    serato.mkdir()
+    music = tmp_path / "Music"
+    first = music / "A" / "Track.mp3"
+    second = music / "B" / "Track.mp3"
+    first.parent.mkdir(parents=True)
+    second.parent.mkdir(parents=True)
+    first.write_bytes(b"first")
+    second.write_bytes(b"second")
+
+    result = duplicate_repair(
+        serato, music, first, (first, second), backup_limit=10
+    )
+
+    assert result["applied"] is False
+    assert result["database_v2_modified"] is False
+    assert second.read_bytes() == b"second"
+    assert not second.is_symlink()