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/views.py

@@ -11,10 +11,13 @@ from finance.serializers import (
     CheckoutSessionSerializer,
     InvoiceSerializer,
     PaymentSerializer,
+    PortalSessionSerializer,
 )
 from finance.services.stripe_service import (
     StripeNotConfiguredError,
+    create_billing_portal_session,
     create_checkout_session,
+    resolve_stripe_customer_id,
 )
 from finance.services.webhooks import dispatch_stripe_event
 
@@ -72,6 +75,48 @@ class CreateCheckoutSessionView(APIView):
         )
 
 
+class CreateBillingPortalSessionView(APIView):
+    """Create a Stripe Customer Portal session and return the hosted URL."""
+
+    def post(self, request):
+        serializer = PortalSessionSerializer(data=request.data)
+        serializer.is_valid(raise_exception=True)
+
+        customer_id = resolve_stripe_customer_id(user=request.user)
+        if not customer_id:
+            return Response(
+                {
+                    "detail": (
+                        "No Stripe customer found for this account. "
+                        "Complete Checkout first to manage billing."
+                    )
+                },
+                status=status.HTTP_400_BAD_REQUEST,
+            )
+
+        try:
+            session = create_billing_portal_session(
+                customer_id=customer_id,
+                return_url=serializer.validated_data.get("return_url"),
+            )
+        except StripeNotConfiguredError as exc:
+            return Response(
+                {"detail": str(exc)},
+                status=status.HTTP_503_SERVICE_UNAVAILABLE,
+            )
+        except stripe.StripeError as exc:
+            logger.exception("Stripe Billing Portal Session creation failed")
+            return Response(
+                {"detail": str(getattr(exc, "user_message", None) or exc)},
+                status=status.HTTP_502_BAD_GATEWAY,
+            )
+
+        return Response(
+            {"portal_url": session.url},
+            status=status.HTTP_201_CREATED,
+        )
+
+
 class InvoiceListView(APIView):
     def get(self, request):
         invoices = Invoice.objects.filter(user=request.user)