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/decorators.py

@@ -57,6 +57,12 @@ class super_admin_required(BasePermissionDecorator):
         return user.is_authenticated and user.is_super_admin()
 
 
+class teacher_admin_required(BasePermissionDecorator):
+    def check_permission(self, request):
+        user = request.user
+        return user.is_authenticated and user.is_teacher_or_above()
+
+
 class admin_role_required(BasePermissionDecorator):
     def check_permission(self, request):
         user = request.user