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/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,
+    )