Add monetization app with RevenueCat webhooks alongside Stripe.

Rename finance → monetization (keep finance_* tables via app label), add
RevenueCat webhook + ledger upserts so store IAP syncs subscriptions and
billing history like Stripe. Companion to chat_web_app#100 / #68.

llm_be/monetization/tests/test_models.py

@@ -0,0 +1,66 @@
+"""Tests for finance Invoice / Payment models and admin registration."""
+
+from django.contrib import admin
+from django.test import TestCase
+
+from chat_backend.tests.factories import make_company, make_user
+from monetization.models import Invoice, Payment
+
+
+class InvoicePaymentModelTestCase(TestCase):
+    def setUp(self):
+        self.company = make_company()
+        self.user = make_user(company=self.company)
+
+    def test_create_invoice_and_payment(self):
+        invoice = Invoice.objects.create(
+            user=self.user,
+            company=self.company,
+            status=Invoice.Status.OPEN,
+            amount_due=1000,
+            currency="usd",
+            stripe_checkout_session_id="cs_test_1",
+        )
+        payment = Payment.objects.create(
+            user=self.user,
+            company=self.company,
+            invoice=invoice,
+            amount=1000,
+            currency="usd",
+            status=Payment.Status.PENDING,
+            stripe_payment_intent_id="pi_test_1",
+        )
+        self.assertEqual(invoice.provider, Invoice.Provider.STRIPE)
+        self.assertEqual(payment.invoice_id, invoice.pk)
+        self.assertEqual(Invoice.objects.count(), 1)
+        self.assertEqual(Payment.objects.count(), 1)
+
+    def test_mark_payment_succeeded(self):
+        payment = Payment.objects.create(
+            user=self.user,
+            amount=1000,
+            stripe_payment_intent_id="pi_test_2",
+        )
+        payment.mark_succeeded()
+        payment.refresh_from_db()
+        self.assertEqual(payment.status, Payment.Status.SUCCEEDED)
+        self.assertIsNotNone(payment.paid_at)
+
+    def test_unique_stripe_checkout_session_id(self):
+        Invoice.objects.create(
+            user=self.user,
+            stripe_checkout_session_id="cs_unique",
+            amount_due=1000,
+        )
+        with self.assertRaises(Exception):
+            Invoice.objects.create(
+                user=self.user,
+                stripe_checkout_session_id="cs_unique",
+                amount_due=1000,
+            )
+
+
+class FinanceAdminRegistrationTestCase(TestCase):
+    def test_invoice_and_payment_registered(self):
+        self.assertIn(Invoice, admin.site._registry)
+        self.assertIn(Payment, admin.site._registry)