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/resolver.ts

@@ -108,6 +108,23 @@ export class SubscriptionType implements Partial<Subscription> {
   @Field(() => Date)
   updatedAt!: Date;
 
+  // read-only fields for display purpose
+  // provider: 'stripe' | 'revenuecat'
+  @Field(() => String, {
+    nullable: true,
+    description:
+      'Payment provider of this subscription. Read-only. One of: stripe | revenuecat',
+  })
+  provider?: string | null;
+
+  // iapStore: 'app_store' | 'play_store' | null when provider is stripe
+  @Field(() => String, {
+    nullable: true,
+    description:
+      'If provider is revenuecat, indicates underlying store. Read-only. One of: app_store | play_store',
+  })
+  iapStore?: string | null;
+
   // deprecated fields
   @Field(() => String, {
     name: 'id',