Persist file categories on the index and classify archives and unknown types during idle maintenance.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/Explorer.Analysis.Tests/AnalysisTests.cs

@@ -39,6 +39,8 @@ public class AnalysisTests
         Assert.Equal("a.mkv", files[0].Name);
         var types = await analysis.UsageByExtensionAsync(source.Id, null);
         Assert.Contains(types, t => t.Extension == "mkv" && t.TotalSize == 200);
+        var cats = await analysis.UsageByCategoryAsync(source.Id, null);
+        Assert.Contains(cats, c => c.Category == "Video" && c.TotalSize == 200);
         var drill = await analysis.DrilldownAsync(root.Id);
         Assert.Contains(drill, e => e.Name == "Movies");
         Assert.Equal("Movies", dirs[0].PathRel);