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

llm_be/finance/urls.py

@@ -0,0 +1,31 @@
+from django.urls import path
+
+from finance.views import (
+    CreateCheckoutSessionView,
+    InvoiceListView,
+    PaymentListView,
+    StripeWebhookView,
+)
+
+urlpatterns = [
+    path(
+        "checkout/",
+        CreateCheckoutSessionView.as_view(),
+        name="finance_checkout",
+    ),
+    path(
+        "invoices/",
+        InvoiceListView.as_view(),
+        name="finance_invoices",
+    ),
+    path(
+        "payments/",
+        PaymentListView.as_view(),
+        name="finance_payments",
+    ),
+    path(
+        "webhooks/stripe/",
+        StripeWebhookView.as_view(),
+        name="finance_stripe_webhook",
+    ),
+]