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.

.env.prod.example

@@ -87,10 +87,12 @@ OAUTH_CALLBACK_BASE_URL=https://chatbackend.aimloperations.com
 #   https://chatbackend.aimloperations.com/api/drive/webhooks/microsoft/
 # Scheduled sync (cron / server-infra job): `python manage.py sync_drive_connections`
 
-# Stripe / finance
+# Stripe / monetization
 STRIPE_SECRET_KEY=replace-with-stripe-secret-key
 STRIPE_PUBLISHABLE_KEY=replace-with-stripe-publishable-key
 STRIPE_WEBHOOK_SECRET=replace-with-stripe-webhook-secret
+REVENUECAT_WEBHOOK_SECRET=replace-with-revenuecat-webhook-auth-token
+# REVENUECAT_PRODUCT_PLAN_MAP={"hesychia_founders_monthly":"founders"}
 # Optional: pre-created Stripe Price ID. When empty, Checkout uses
 # SUBSCRIPTION_PRICE_* from settings.py ($10 USD / month by default).
 STRIPE_PRICE_ID=