Add finance app with Stripe Checkout subscriptions (#21) (#23)

## Summary
- Closes #21 — new Django `finance` app with Stripe as payment provider
- Subscription price defaults to **$10 USD / month** via `SUBSCRIPTION_PRICE_AMOUNT_CENTS = 1000` in `settings.py` (env-overridable)
- Persists **Invoice** and **Payment** rows; both registered in Django admin (with payment inline on invoices)
- Checkout Session API redirects users to Stripe hosted payment; webhook verifies signatures and upserts ledger idempotently

## API
- `POST /api/finance/checkout/` — JWT auth → `{ checkout_url, session_id }`
- `GET /api/finance/invoices/` / `GET /api/finance/payments/` — own records
- `POST /api/finance/webhooks/stripe/` — Stripe signature-verified webhook

## Config
Documented in `.env.example` / `.env.prod.example`:
`STRIPE_SECRET_KEY`, `STRIPE_PUBLISHABLE_KEY`, `STRIPE_WEBHOOK_SECRET`, optional `STRIPE_PRICE_ID`, `FRONTEND_BASE_URL`

## Test plan
- [x] `uv run python manage.py test finance` (17 tests)
- [ ] Set Stripe test keys locally; create checkout session; complete payment in Stripe test mode
- [ ] Confirm Invoice/Payment appear in `/admin/`
- [ ] Point Stripe webhook to `/api/finance/webhooks/stripe/` and verify `checkout.session.completed` / `invoice.paid`Reviewed-on: #23

.env.prod.example

@@ -45,6 +45,17 @@ EMAIL_USE_TLS=true
 # Captcha
 CAPTCHA_SECRET_KEY=replace-with-captcha-secret
 
+# Stripe / finance
+STRIPE_SECRET_KEY=replace-with-stripe-secret-key
+STRIPE_PUBLISHABLE_KEY=replace-with-stripe-publishable-key
+STRIPE_WEBHOOK_SECRET=replace-with-stripe-webhook-secret
+# Optional: pre-created Stripe Price ID. When empty, Checkout uses
+# SUBSCRIPTION_PRICE_* from settings.py ($10 USD / month by default).
+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
+
 # Gunicorn / ASGI (UvicornWorker for WebSockets)
 GUNICORN_WORKERS=2
 GUNICORN_BIND=0.0.0.0:8000