feat: add Teacher Admin role to four-tier permission system

Introduces a four-tier role system: Regular User → Student Admin →
Teacher Admin → Super Admin. Teacher Admin can manage own contests,
problemsets, and view classroom data. Student Admin (renamed from Admin)
retains problem management only.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

account/migrations/0007_rename_admin_to_student_admin.py

@@ -0,0 +1,27 @@
+# Generated by Django 6.0.4 on 2026-06-03 00:08
+
+from django.db import migrations
+
+
+def rename_admin_to_student_admin(apps, schema_editor):
+    User = apps.get_model("account", "User")
+    User.objects.filter(admin_type="Admin").update(admin_type="Student Admin")
+
+
+def rename_student_admin_to_admin(apps, schema_editor):
+    User = apps.get_model("account", "User")
+    User.objects.filter(admin_type="Student Admin").update(admin_type="Admin")
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("account", "0006_alter_user_admin_type"),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            rename_admin_to_student_admin,
+            rename_student_admin_to_admin,
+        ),
+    ]