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/consumers_graph.py

@@ -39,7 +39,7 @@ from .utils import (
     is_heartbeat_payload,
     normalize_user_message,
 )
-from finance.services.quotas import FeatureNotAllowed, QuotaExceeded, check_generation_allowed
+from monetization.services.quotas import FeatureNotAllowed, QuotaExceeded, check_generation_allowed
 
 logger = logging.getLogger(__name__)
 
@@ -74,7 +74,7 @@ def enforce_generation_gates(user, feature="text_generation"):
 
 @database_sync_to_async
 def enforce_feature_gate(user, feature):
-    from finance.services.quotas import assert_feature_allowed
+    from monetization.services.quotas import assert_feature_allowed
 
     assert_feature_allowed(user, feature)