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).

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 |