Use export date from filename for archive subdirectory

Extract the date from filenames like "2026-01-20.json" instead of
using the current date, so archives are organized by export date
rather than processing date.

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

src/DiscordArchiveManager/Services/ArchiveService.cs

@@ -28,8 +28,8 @@ public class ArchiveService
         var jsonDirectory = Path.GetDirectoryName(jsonFilePath)!;
         var filesDirectory = GetFilesDirectoryPath(jsonFilePath);
 
-        // Create archive subdirectory based on date
-        var archiveSubdir = DateTime.Now.ToString("yyyy-MM-dd");
+        // Create archive subdirectory based on export date from filename
+        var archiveSubdir = GetExportDateFromFilename(jsonFileName) ?? DateTime.Now.ToString("yyyy-MM-dd");
         var archivePath = Path.Combine(archiveRoot, archiveSubdir);
         Directory.CreateDirectory(archivePath);
 
@@ -152,6 +152,26 @@ public class ArchiveService
         }
     }
 
+    /// <summary>
+    /// Extracts the export date from a filename like "2026-01-20.json".
+    /// Returns null if the filename doesn't match the expected pattern.
+    /// </summary>
+    private static string? GetExportDateFromFilename(string filename)
+    {
+        var nameWithoutExtension = Path.GetFileNameWithoutExtension(filename);
+        if (DateTime.TryParseExact(nameWithoutExtension, "yyyy-MM-dd", null, System.Globalization.DateTimeStyles.None, out _))
+        {
+            return nameWithoutExtension;
+        }
+        // Handle filenames with suffix like "2026-01-20_1.json"
+        var parts = nameWithoutExtension.Split('_');
+        if (parts.Length > 0 && DateTime.TryParseExact(parts[0], "yyyy-MM-dd", null, System.Globalization.DateTimeStyles.None, out _))
+        {
+            return parts[0];
+        }
+        return null;
+    }
+
     /// <summary>
     /// Moves a file, falling back to copy+delete for cross-device moves.
     /// </summary>