feat: import leftover student fields into notes and show them on the roster

Store leftover CSV columns on hidden student notes, move catalog import to /events/import, and persist Students index columns from Chapter Settings.

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

WebApp/Services/IStudentNotesImportSaveService.cs

@@ -0,0 +1,22 @@
+using Core.Models;
+
+namespace WebApp.Services;
+
+/// <summary>
+/// Persists merged student field notes.
+/// </summary>
+public interface IStudentNotesImportSaveService
+{
+    Task<StudentNotesImportSaveResult> SaveAsync(
+        StudentNotesImportResult parseResult,
+        CancellationToken cancellationToken = default);
+}
+
+public class StudentNotesImportSaveResult
+{
+    public int NotesCreated { get; set; }
+
+    public int NotesUpdated { get; set; }
+
+    public int StudentsUnchanged { get; set; }
+}