Add Stripe Customer Portal session API for account billing (#35)

## Summary
- Companion to [chat_web_app#33](ai_ml_operations/chat_web_app#33) (Account billing + Customer Portal)
- Follow-on from finance MVP [#21](#21): add authenticated `POST /api/finance/portal/` that creates a Stripe Billing Portal session and returns `portal_url`
- Resolve Stripe customer from the user's latest `Invoice.stripe_customer_id`; return `400` when missing (user must complete Checkout first)
- Document `STRIPE_PORTAL_RETURN_URL` (default `{FRONTEND_BASE_URL}/account/`) in settings + env examples

## Test plan
- [ ] `manage.py test finance.tests.test_portal finance.tests.test_checkout`
- [ ] Authenticated portal create with invoice that has `stripe_customer_id` → `201` + `portal_url`
- [ ] No customer / unpaid user → `400` with clear detail
- [ ] Missing `STRIPE_SECRET_KEY` → `503`
- [ ] Unauthenticated → `401`
- [ ] Custom `return_url` in body overrides default portal return URLReviewed-on: #35

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(),