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

README.md

@@ -93,7 +93,8 @@ with `COMPOSE_DATABASE_URL` if needed.
 | `ENABLE_ACCOUNT_REGISTRATION` | `false` | optional | Self-serve sign-up; keep false until ready |
 | `STRIPE_SECRET_KEY` / `STRIPE_PUBLISHABLE_KEY` / `STRIPE_WEBHOOK_SECRET` | empty | yes for billing | Stripe API + webhook |
 | `STRIPE_PRICE_ID` | empty | optional | Pre-created Price; else `$10/mo` from settings |
-| `FRONTEND_BASE_URL` | `http://localhost:3000` | set in prod/beta | Checkout success/cancel + OAuth return |
+| `FRONTEND_BASE_URL` | `http://localhost:3000` | set in prod/beta | Checkout success/cancel, portal return, OAuth return |
+| `STRIPE_PORTAL_RETURN_URL` | `{FRONTEND}/account/` | optional | Stripe Customer Portal return URL |
 | `CORS_ALLOWED_ORIGINS` | local + chat FE (+ beta FE default) | set in prod/beta | Frontend origin(s) |
 | `USE_TLS_PROXY` | false (dev) | true behind NPM | Sets `SECURE_PROXY_SSL_HEADER` |
 | `GUNICORN_WORKERS` / `GUNICORN_BIND` | 2 / `0.0.0.0:8000` | optional | Entrypoint |