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>

Core/Services/IStudentNotesImportService.cs

@@ -0,0 +1,15 @@
+using Core.Entities;
+using Core.Models;
+
+namespace Core.Services;
+
+/// <summary>
+/// Parses student field CSVs and merges them into markdown notes.
+/// </summary>
+public interface IStudentNotesImportService
+{
+    StudentNotesImportResult Parse(
+        Stream stream,
+        ICollection<Student> students,
+        IReadOnlyDictionary<int, string?> existingNotesByStudentId);
+}