feat: sync rcat data (#13628)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* RevenueCat support: public webhook endpoint, webhook handler/service,
nightly reconciliation and per-user sync; subscriptions now expose
provider and iapStore; new user-facing error for App Store/Play-managed
subscriptions.
* **Chores**
* Multi-provider subscription schema (Provider, IapStore); Stripe
credentials moved into payment.stripe (top-level apiKey/webhookKey
deprecated); new payment.revenuecat config and defaults added.
* **Tests**
  * Comprehensive RevenueCat integration test suite and snapshots.
* **Documentation**
  * Admin config descriptions updated with deprecation guidance.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/payment/manager/common.ts

@@ -30,6 +30,9 @@ export interface Subscription {
   trialEnd: Date | null;
   nextBillAt: Date | null;
   canceledAt: Date | null;
+  // read-only metadata for IAP integration
+  provider?: string | null;
+  iapStore?: string | null;
 }
 
 export interface Invoice {