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/monetization/signals.py

@@ -0,0 +1,8 @@
+"""Finance app signals."""
+
+
+def seed_plans_on_migrate(sender, **kwargs):
+    """Ensure the subscription catalog exists after migrate."""
+    from monetization.services.plans import seed_subscription_plans
+
+    seed_subscription_plans(update_existing=False)