fix(reward-service): 修复 Prisma schema 缺少 url 字段

添加 datasource url 配置，解决构建时 Prisma generate 失败

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Co-Authored-By: Claude <noreply@anthropic.com>

backend/services/reward-service/prisma/schema.prisma

@@ -4,6 +4,7 @@ generator client {
 
 datasource db {
   provider = "postgresql"
+  url      = env("DATABASE_URL")
 }
 
 // ============================================