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/StudentNotesImportService.cs

@@ -0,0 +1,22 @@
+using Core.Entities;
+using Core.Models;
+using Core.Parsers;
+
+namespace Core.Services;
+
+/// <summary>
+/// Wraps <see cref="StudentNotesFieldParser"/> for stream-based import.
+/// </summary>
+public class StudentNotesImportService : IStudentNotesImportService
+{
+    /// <inheritdoc />
+    public StudentNotesImportResult Parse(
+        Stream stream,
+        ICollection<Student> students,
+        IReadOnlyDictionary<int, string?> existingNotesByStudentId)
+    {
+        var reader = new StreamReader(stream, leaveOpen: true);
+        using var parser = new StudentNotesFieldParser(reader);
+        return parser.Parse(students, existingNotesByStudentId);
+    }
+}