Add wallet balance to User model and implement Atoll/Island management

- Added `wallet_balance` field to the User model.
- Updated UserAdmin to include `wallet_balance` in the admin interface.
- Created serializers and views for Atoll and Island management.
- Implemented endpoints for listing, creating, and updating Atolls and Islands.
- Enhanced payment processing with UUIDs for Payment and Topup models.
- Added migration files for new fields and constraints.
- Improved error handling and validation in various views.
- Updated email templates for better responsiveness and SEO.

billing/urls.py

@@ -0,0 +1,8 @@
+# billing/urls.py
+from django.urls import path
+from .views import CreatePaymentView, VerifyPaymentView
+
+urlpatterns = [
+    path("create-payment/", CreatePaymentView.as_view(), name="create-payment"),
+    path("verify-payment/", VerifyPaymentView.as_view(), name="verify-payment"),
+]