Monetization app + RevenueCat webhooks (store IAP ledger) (#69)

## Summary
- Rename `finance` → **`monetization`** Django app (keep `finance_*` tables via `label = "finance"`)
- Add `services/stripe.py` + `services/revenuecat.py`; RevenueCat webhook upserts **subscription + Invoice/Payment** (billing history parity with Stripe)
- Mount `/api/monetization/` + keep `/api/finance/` alias
- Extend `Source`/`Provider` with `revenuecat`; product→plan mapping via `revenuecat_product_id` / `REVENUECAT_PRODUCT_PLAN_MAP`

Closes #68. Companion to [chat_web_app#100](ai_ml_operations/chat_web_app#100).

## Test plan
- [x] `manage.py test monetization.tests` (54 OK)
- [x] Smoke `chat_backend.tests.test_views_documents` + `test_oauth`
- [ ] Deploy: set `REVENUECAT_WEBHOOK_SECRET`; point RC webhook at `/api/finance/webhooks/revenuecat/`
- [ ] Map store product IDs on `SubscriptionPlan.revenuecat_product_id` (or env JSON map)
- [ ] Sandbox INITIAL_PURCHASE → subscription `source=revenuecat` + invoice in `/finance/invoices/`Reviewed-on: #69

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",
+    ),
+]