fix(backend): save data exports as data-export.zip

change the random base 64 to a directory rather than part of the
filename, so that users downloading their exports aren't greeted with a
completely incomprehensible file in their downloads folder

Foxnouns.Backend/Services/DataCleanupService.cs

@@ -128,5 +128,5 @@ public class DataCleanupService(
     }
 
     private static string ExportPath(Snowflake userId, string b64) =>
-        $"data-exports/{userId}/{b64}.zip";
+        $"data-exports/{userId}/{b64}/data-export.zip";
 }