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

.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=