feat: add pending student registration flow and pricing APIs.

Support public class lookup, user provisioning, installment pricing, and admin approval before final enrollment.

components/classes/classService.js

@@ -202,4 +202,19 @@ const getPublicClasses = async (query = {}) => {
   return { data: items.map(enrichClassForDisplay), meta: calculateMeta(total, page, limit) };
 };
 
-module.exports = { getAll, getOne, create, update, remove, registerUsers, removeUser, getMyClasses, getPublicClasses };
+const getPublicOne = async (id) => {
+  const cls = await Class.findOne({
+    _id: id,
+    isActive: { $ne: false },
+    showOnFrontend: { $ne: false }
+  })
+    .select('name course professor capacity tuitionFee hasDiscount discount freeSpots startDate endDate days startTime endTime isActive')
+    .populate({ path: 'course', select: 'title type description sectionCount hoursPerSection price' })
+    .populate({ path: 'professor', select: 'name surname' })
+    .lean();
+
+  if (!cls) throw new AppError('CLASS_NOT_FOUND', null, 'کلاس یافت نشد یا برای ثبت‌نام در دسترس نیست.');
+  return enrichClassForDisplay(cls);
+};
+
+module.exports = { getAll, getOne, create, update, remove, registerUsers, removeUser, getMyClasses, getPublicClasses, getPublicOne };