feat: add optional student nickname for informal display

Keep legal first and last names for formal lists; show DisplayFirstName on teams, calendars, and import matching so two Josiahs can be told apart.

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

Data/Migrations/20260911190000_AddStudentNickname.cs

@@ -0,0 +1,33 @@
+using Data;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Data.Migrations
+{
+    /// <inheritdoc />
+    [DbContext(typeof(AppDbContext))]
+    [Migration("20260911190000_AddStudentNickname")]
+    public partial class AddStudentNickname : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "Nickname",
+                table: "Students",
+                type: "TEXT",
+                maxLength: 100,
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "Nickname",
+                table: "Students");
+        }
+    }
+}