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/admin.py

@@ -0,0 +1,86 @@
+from django.contrib import admin
+
+from finance.models import Invoice, Payment
+
+
+class PaymentInline(admin.TabularInline):
+    model = Payment
+    extra = 0
+    readonly_fields = (
+        "provider",
+        "status",
+        "amount",
+        "currency",
+        "stripe_payment_intent_id",
+        "stripe_charge_id",
+        "paid_at",
+        "failure_message",
+        "created",
+        "last_modified",
+    )
+    can_delete = False
+    show_change_link = True
+
+
+@admin.register(Invoice)
+class InvoiceAdmin(admin.ModelAdmin):
+    list_display = (
+        "id",
+        "user",
+        "company",
+        "provider",
+        "status",
+        "amount_due",
+        "amount_paid",
+        "currency",
+        "period_start",
+        "period_end",
+        "stripe_invoice_id",
+        "stripe_checkout_session_id",
+        "created",
+    )
+    list_filter = ("provider", "status", "currency")
+    search_fields = (
+        "user__email",
+        "user__username",
+        "company__name",
+        "stripe_invoice_id",
+        "stripe_checkout_session_id",
+        "stripe_subscription_id",
+        "stripe_customer_id",
+        "description",
+    )
+    readonly_fields = ("created", "last_modified")
+    raw_id_fields = ("user", "company")
+    inlines = [PaymentInline]
+    date_hierarchy = "created"
+
+
+@admin.register(Payment)
+class PaymentAdmin(admin.ModelAdmin):
+    list_display = (
+        "id",
+        "user",
+        "company",
+        "invoice",
+        "provider",
+        "status",
+        "amount",
+        "currency",
+        "stripe_payment_intent_id",
+        "stripe_charge_id",
+        "paid_at",
+        "created",
+    )
+    list_filter = ("provider", "status", "currency")
+    search_fields = (
+        "user__email",
+        "user__username",
+        "company__name",
+        "stripe_payment_intent_id",
+        "stripe_charge_id",
+        "invoice__stripe_invoice_id",
+    )
+    readonly_fields = ("created", "last_modified")
+    raw_id_fields = ("user", "company", "invoice")
+    date_hierarchy = "created"