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/schema.prisma

@@ -749,6 +749,16 @@ model Subscription {
   stripeSubscriptionId String?   @unique @map("stripe_subscription_id")
   // stripe schedule id
   stripeScheduleId     String?   @map("stripe_schedule_id") @db.VarChar
+  // subscription provider: stripe or revenuecat
+  provider             Provider  @default(stripe)
+  // iap store for revenuecat subscriptions
+  iapStore             IapStore? @map("iap_store")
+  // revenuecat entitlement name like "Pro" / "AI"
+  rcEntitlement        String?   @map("rc_entitlement") @db.VarChar
+  // revenuecat product id like "app.affine.pro.Annual"
+  rcProductId          String?   @map("rc_product_id") @db.VarChar
+  // external reference, appstore originalTransactionId or play purchaseToken
+  rcExternalRef        String?   @map("rc_external_ref") @db.VarChar
   // subscription.status, active/past_due/canceled/unpaid...
   status               String    @db.VarChar(20)
   // subscription.current_period_start
@@ -770,6 +780,16 @@ model Subscription {
   @@map("subscriptions")
 }
 
+enum Provider {
+  stripe
+  revenuecat
+}
+
+enum IapStore {
+  app_store
+  play_store
+}
+
 model Invoice {
   stripeInvoiceId             String   @id @map("stripe_invoice_id")
   targetId                    String   @map("target_id") @db.VarChar