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/Notes/StudentNoteFieldDefaults.cs

@@ -0,0 +1,14 @@
+namespace Core.Notes;
+
+/// <summary>
+/// Default imported-field names used for Students index columns and the CSV template.
+/// </summary>
+public static class StudentNoteFieldDefaults
+{
+    public static readonly string[] IndexColumns =
+    [
+        "Interview Time",
+        "Application",
+        "Club Permission Slip"
+    ];
+}