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/chat_backend/serializers.py

@@ -72,8 +72,8 @@ class CustomUserSerializer(serializers.ModelSerializer):
         extra_kwargs = {"password": {"write_only": True}}
 
     def get_subscription(self, obj):
-        from finance.services.plans import needs_checkout, plan_to_dict
-        from finance.models import UserSubscription
+        from monetization.services.plans import needs_checkout, plan_to_dict
+        from monetization.models import UserSubscription
 
         try:
             sub = obj.subscription
@@ -122,7 +122,7 @@ class SelfServeRegistrationSerializer(serializers.Serializer):
         return email
 
     def create(self, validated_data):
-        from finance.services.plans import try_redeem_backer_email
+        from monetization.services.plans import try_redeem_backer_email
 
         email = validated_data["email"]
         password = validated_data["password"]