Accept bounded StoreKit signing clock skew

Allow legitimate App Store transaction timestamps to differ within the existing verification tolerance while preserving rejection beyond that boundary.

src/main/kotlin/com/osglab/account/features/storekit/services/StoreKitService.kt

@@ -123,7 +123,7 @@ class StoreKitService(
         }
         val now = clock.instant()
         if (
-            transaction.purchasedAt > transaction.signedAt ||
+            transaction.purchasedAt > transaction.signedAt.plus(MAX_CLOCK_SKEW) ||
             transaction.signedAt > now.plus(MAX_CLOCK_SKEW)
         ) {
             throw StoreKitPurchaseConflict()

src/test/kotlin/com/osglab/account/features/storekit/StoreKitServiceTest.kt

@@ -33,14 +33,16 @@ class StoreKitServiceTest : FunSpec({
         accountToken: UUID = userId,
         productId: String = product.productId,
         revokedAt: Instant? = null,
+        purchasedAt: Instant = now.minusSeconds(10),
+        signedAt: Instant = now.minusSeconds(5),
     ) = VerifiedStoreKitTransaction(
         transactionId = transactionId,
         originalTransactionId = transactionId,
         appAccountToken = accountToken,
         productId = productId,
         environment = StoreKitEnvironment.SANDBOX,
-        purchasedAt = now.minusSeconds(10),
-        signedAt = now.minusSeconds(5),
+        purchasedAt = purchasedAt,
+        signedAt = signedAt,
         revokedAt = revokedAt,
     )
 
@@ -115,6 +117,33 @@ class StoreKitServiceTest : FunSpec({
         store.ledger shouldHaveSize 0
     }
 
+    test("small App Store signing clock skew is accepted") {
+        val store = TestBillingStore()
+        val verified = transaction(
+            purchasedAt = now.minusSeconds(10),
+            signedAt = now.minusSeconds(70),
+        )
+
+        val result = service(store, verified).submit(userId, signedTransaction)
+
+        result.balanceAfter shouldBeExactly 3_000
+        store.ledger shouldHaveSize 1
+    }
+
+    test("App Store signing clock skew beyond the tolerance is rejected") {
+        val store = TestBillingStore()
+        val verified = transaction(
+            purchasedAt = now.minusSeconds(10),
+            signedAt = now.minusSeconds(311),
+        )
+
+        shouldThrow<StoreKitPurchaseConflict> {
+            service(store, verified).submit(userId, signedTransaction)
+        }
+
+        store.ledger shouldHaveSize 0
+    }
+
     test("unknown or revoked products never grant credits") {
         val unknownStore = TestBillingStore()
         shouldThrow<StoreKitPurchaseConflict> {