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/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)