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/serializers.py

@@ -0,0 +1,51 @@
+from rest_framework import serializers
+
+from finance.models import Invoice, Payment
+
+
+class InvoiceSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = Invoice
+        fields = [
+            "id",
+            "provider",
+            "status",
+            "currency",
+            "amount_due",
+            "amount_paid",
+            "period_start",
+            "period_end",
+            "stripe_invoice_id",
+            "stripe_checkout_session_id",
+            "stripe_subscription_id",
+            "hosted_invoice_url",
+            "description",
+            "created",
+            "last_modified",
+        ]
+        read_only_fields = fields
+
+
+class PaymentSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = Payment
+        fields = [
+            "id",
+            "invoice",
+            "provider",
+            "status",
+            "currency",
+            "amount",
+            "stripe_payment_intent_id",
+            "stripe_charge_id",
+            "paid_at",
+            "failure_message",
+            "created",
+            "last_modified",
+        ]
+        read_only_fields = fields
+
+
+class CheckoutSessionSerializer(serializers.Serializer):
+    success_url = serializers.URLField(required=False, allow_blank=False)
+    cancel_url = serializers.URLField(required=False, allow_blank=False)