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/apps.py

@@ -0,0 +1,22 @@
+from django.apps import AppConfig
+
+
+class MonetizationConfig(AppConfig):
+    """Paid access: plans, quotas, Stripe, RevenueCat.
+
+    ``label`` stays ``finance`` so existing ``finance_*`` tables and
+    ``django_migrations`` / contenttypes rows keep working after the package
+    rename from ``finance`` → ``monetization``.
+    """
+
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "monetization"
+    label = "finance"
+    verbose_name = "Monetization"
+
+    def ready(self):
+        from django.db.models.signals import post_migrate
+
+        from monetization.signals import seed_plans_on_migrate
+
+        post_migrate.connect(seed_plans_on_migrate, sender=self)