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/urls.py

@@ -0,0 +1,55 @@
+from django.urls import path
+
+from monetization.views import (
+    CreateBillingPortalSessionView,
+    CreateCheckoutSessionView,
+    InvoiceListView,
+    PaymentListView,
+    PlanListView,
+    RevenueCatWebhookView,
+    StripeWebhookView,
+    SubscriptionMeView,
+)
+
+urlpatterns = [
+    path(
+        "checkout/",
+        CreateCheckoutSessionView.as_view(),
+        name="finance_checkout",
+    ),
+    path(
+        "portal/",
+        CreateBillingPortalSessionView.as_view(),
+        name="finance_portal",
+    ),
+    path(
+        "invoices/",
+        InvoiceListView.as_view(),
+        name="finance_invoices",
+    ),
+    path(
+        "payments/",
+        PaymentListView.as_view(),
+        name="finance_payments",
+    ),
+    path(
+        "plans/",
+        PlanListView.as_view(),
+        name="finance_plans",
+    ),
+    path(
+        "subscription/",
+        SubscriptionMeView.as_view(),
+        name="finance_subscription_me",
+    ),
+    path(
+        "webhooks/stripe/",
+        StripeWebhookView.as_view(),
+        name="finance_stripe_webhook",
+    ),
+    path(
+        "webhooks/revenuecat/",
+        RevenueCatWebhookView.as_view(),
+        name="finance_revenuecat_webhook",
+    ),
+]