fix: 后端资金安全修复 — 金额覆盖、过期订单、退款原子性等 9 项

- confirmPayment 不再覆盖 amount，实付金额写入 payAmount
- EXPIRED 订单增加 5 分钟宽限窗口
- 退款流程先扣余额再退款，失败可回滚
- 支付宝签名过滤 sign_type
- executeRecharge 使用 CAS 更新
- createOrder rechargeCode 事务保护
- EasyPay/Sub2API client 添加 10s 超时
- db.ts 统一从 getEnv() 获取 DATABASE_URL
- 添加 paymentType+paidAt 复合索引

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/db.ts

@@ -1,10 +1,11 @@
 import { PrismaClient } from '@prisma/client';
 import { PrismaPg } from '@prisma/adapter-pg';
+import { getEnv } from '@/lib/config';
 
 const globalForPrisma = globalThis as unknown as { prisma: PrismaClient };
 
 function createPrismaClient() {
-  const connectionString = process.env.DATABASE_URL ?? 'postgresql://localhost:5432/sub2apipay';
+  const connectionString = getEnv().DATABASE_URL;
   const adapter = new PrismaPg({ connectionString });
   return new PrismaClient({ adapter });
 }