Add Stripe Customer Portal session API for account billing.

Expose POST /api/finance/portal/ so authenticated users can open Stripe's hosted portal for plan changes, payment methods, and cancellations (chat_web_app#33 companion).

llm_be/finance/urls.py

@@ -1,6 +1,7 @@
 from django.urls import path
 
 from finance.views import (
+    CreateBillingPortalSessionView,
     CreateCheckoutSessionView,
     InvoiceListView,
     PaymentListView,
@@ -13,6 +14,11 @@ urlpatterns = [
         CreateCheckoutSessionView.as_view(),
         name="finance_checkout",
     ),
+    path(
+        "portal/",
+        CreateBillingPortalSessionView.as_view(),
+        name="finance_portal",
+    ),
     path(
         "invoices/",
         InvoiceListView.as_view(),