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

.env.prod.example

@@ -70,6 +70,7 @@ STRIPE_PRICE_ID=
 FRONTEND_BASE_URL=https://chat.aimloperations.com
 # STRIPE_CHECKOUT_SUCCESS_URL=https://chat.aimloperations.com/billing/success?session_id={CHECKOUT_SESSION_ID}
 # STRIPE_CHECKOUT_CANCEL_URL=https://chat.aimloperations.com/billing/cancel
+# STRIPE_PORTAL_RETURN_URL=https://chat.aimloperations.com/account/
 
 # Gunicorn / ASGI (UvicornWorker for WebSockets)
 GUNICORN_WORKERS=2