perf: 添加 paid_at 索引优化 dashboard 查询性能

prisma/migrations/20260304000000_add_paid_at_index/migration.sql

@@ -0,0 +1,2 @@
+-- CreateIndex
+CREATE INDEX "orders_paid_at_idx" ON "orders"("paid_at");

prisma/schema.prisma

@@ -46,6 +46,7 @@ model Order {
   @@index([status])
   @@index([expiresAt])
   @@index([createdAt])
+  @@index([paidAt])
   @@map("orders")
 }