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/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,
+    )