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/StudentNotesImportPlan.cs

@@ -0,0 +1,54 @@
+using Core.Models;
+
+namespace Core.Services;
+
+/// <summary>
+/// Decides which parsed student notes should be created or updated.
+/// Unchanged merges are omitted so a re-import does not write history.
+/// </summary>
+public static class StudentNotesImportPlan
+{
+    /// <summary>
+    /// Builds persist actions from a parse result. Only matches with field changes are included.
+    /// </summary>
+    public static IReadOnlyList<StudentNotePersistAction> Create(
+        StudentNotesImportResult parseResult,
+        IReadOnlySet<int> studentIdsWithExistingNotes)
+    {
+        ArgumentNullException.ThrowIfNull(parseResult);
+        ArgumentNullException.ThrowIfNull(studentIdsWithExistingNotes);
+
+        List<StudentNotePersistAction> actions = [];
+        foreach (var match in parseResult.Matches)
+        {
+            if (!match.Merge.Changed)
+                continue;
+
+            actions.Add(new StudentNotePersistAction
+            {
+                StudentId = match.Student.Id,
+                Markdown = match.Merge.Markdown,
+                Kind = studentIdsWithExistingNotes.Contains(match.Student.Id)
+                    ? StudentNotePersistKind.Update
+                    : StudentNotePersistKind.Create
+            });
+        }
+
+        return actions;
+    }
+}
+
+public enum StudentNotePersistKind
+{
+    Create,
+    Update
+}
+
+public class StudentNotePersistAction
+{
+    public required int StudentId { get; init; }
+
+    public required string Markdown { get; init; }
+
+    public required StudentNotePersistKind Kind { get; init; }
+}