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/serializers.py

@@ -49,3 +49,7 @@ class PaymentSerializer(serializers.ModelSerializer):
 class CheckoutSessionSerializer(serializers.Serializer):
     success_url = serializers.URLField(required=False, allow_blank=False)
     cancel_url = serializers.URLField(required=False, allow_blank=False)
+
+
+class PortalSessionSerializer(serializers.Serializer):
+    return_url = serializers.URLField(required=False, allow_blank=False)

llm_be/finance/services/stripe_service.py

@@ -1,4 +1,4 @@
-"""Stripe Checkout session helpers."""
+"""Stripe Checkout and Billing Portal session helpers."""
 
 from __future__ import annotations
 
@@ -7,6 +7,8 @@ from typing import Any
 import stripe
 from django.conf import settings
 
+from finance.models import Invoice
+
 
 class StripeNotConfiguredError(RuntimeError):
     """Raised when Stripe secret key is missing."""
@@ -69,3 +71,27 @@ def create_checkout_session(
         subscription_data={"metadata": metadata},
     )
     return session
+
+
+def resolve_stripe_customer_id(*, user) -> str | None:
+    """Return the most recent Stripe customer id stored on the user's invoices."""
+    return (
+        Invoice.objects.filter(user=user)
+        .exclude(stripe_customer_id="")
+        .order_by("-created")
+        .values_list("stripe_customer_id", flat=True)
+        .first()
+    )
+
+
+def create_billing_portal_session(
+    *,
+    customer_id: str,
+    return_url: str | None = None,
+):
+    """Create a Stripe Customer Portal session for plan/payment/cancel management."""
+    configure_stripe()
+    return stripe.billing_portal.Session.create(
+        customer=customer_id,
+        return_url=return_url or settings.STRIPE_PORTAL_RETURN_URL,
+    )

llm_be/finance/tests/test_portal.py

@@ -0,0 +1,102 @@
+"""Tests for Stripe Customer Portal session API (mocked Stripe SDK)."""
+
+from unittest.mock import MagicMock, patch
+
+from django.test import override_settings
+from django.urls import reverse
+from rest_framework import status
+from rest_framework.test import APITestCase
+
+from chat_backend.tests.factories import make_company, make_user
+from finance.models import Invoice
+
+
+class CreateBillingPortalSessionViewTestCase(APITestCase):
+    def setUp(self):
+        self.company = make_company()
+        self.user = make_user(company=self.company)
+        self.client.force_authenticate(user=self.user)
+        self.url = reverse("finance_portal")
+
+    def _create_invoice_with_customer(self, customer_id="cus_test_abc"):
+        return Invoice.objects.create(
+            user=self.user,
+            company=self.company,
+            amount_due=1000,
+            amount_paid=1000,
+            status=Invoice.Status.PAID,
+            stripe_checkout_session_id="cs_portal_test",
+            stripe_customer_id=customer_id,
+            stripe_subscription_id="sub_test_abc",
+            description="Chat Subscription",
+        )
+
+    @override_settings(
+        STRIPE_SECRET_KEY="sk_test_fake",
+        STRIPE_PORTAL_RETURN_URL="http://localhost:3000/account/",
+    )
+    @patch("finance.services.stripe_service.stripe.billing_portal.Session.create")
+    def test_creates_portal_session(self, mock_create):
+        self._create_invoice_with_customer()
+        mock_session = MagicMock()
+        mock_session.url = "https://billing.stripe.com/p/session/test_portal"
+        mock_create.return_value = mock_session
+
+        response = self.client.post(self.url, {}, format="json")
+
+        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
+        self.assertEqual(
+            response.data["portal_url"],
+            "https://billing.stripe.com/p/session/test_portal",
+        )
+        mock_create.assert_called_once_with(
+            customer="cus_test_abc",
+            return_url="http://localhost:3000/account/",
+        )
+
+    @override_settings(
+        STRIPE_SECRET_KEY="sk_test_fake",
+        STRIPE_PORTAL_RETURN_URL="http://localhost:3000/account/",
+    )
+    @patch("finance.services.stripe_service.stripe.billing_portal.Session.create")
+    def test_accepts_custom_return_url(self, mock_create):
+        self._create_invoice_with_customer()
+        mock_session = MagicMock()
+        mock_session.url = "https://billing.stripe.com/p/session/custom"
+        mock_create.return_value = mock_session
+
+        response = self.client.post(
+            self.url,
+            {"return_url": "http://localhost:3000/account/#billing"},
+            format="json",
+        )
+
+        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
+        self.assertEqual(
+            mock_create.call_args.kwargs["return_url"],
+            "http://localhost:3000/account/#billing",
+        )
+
+    @override_settings(STRIPE_SECRET_KEY="sk_test_fake")
+    def test_no_stripe_customer_returns_400(self):
+        Invoice.objects.create(
+            user=self.user,
+            company=self.company,
+            amount_due=1000,
+            stripe_checkout_session_id="cs_no_customer",
+            stripe_customer_id="",
+        )
+        response = self.client.post(self.url, {}, format="json")
+        self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
+        self.assertIn("No Stripe customer", response.data["detail"])
+
+    @override_settings(STRIPE_SECRET_KEY="")
+    def test_missing_stripe_key_returns_503(self):
+        self._create_invoice_with_customer()
+        response = self.client.post(self.url, {}, format="json")
+        self.assertEqual(response.status_code, status.HTTP_503_SERVICE_UNAVAILABLE)
+
+    def test_unauthenticated_rejected(self):
+        self.client.force_authenticate(user=None)
+        response = self.client.post(self.url, {}, format="json")
+        self.assertEqual(response.status_code, status.HTTP_401_UNAUTHORIZED)

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(),

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)