feat: 统一全链路时区为 UTC+8 (Asia/Shanghai)

- schema.sql created_at DEFAULT 改为 datetime(now,+8 hours)
- Dockerfile runtime 安装 tzdata ENV TZ=Asia/Shanghai 并软链 /etc/localtime
- docker-compose.yml 挂载宿主机 localtime 并传 TZ=Asia/Shanghai
- server index.ts 入口设置 process.env.TZ
- client utils/export 新增 utc8Now/utc8Input/utc8DateStamp/utc8DateTimeStamp
- Activities 默认活动开始/结束时间改用 UTC+8
- 导出文件名日期戳和报告生成时间统一 utc8DateStamp/utc8DateTimeStamp

docker-compose.yml

@@ -8,7 +8,9 @@ services:
       - "3000:3000"
     volumes:
       - ./server/data:/app/server/data
+      - /etc/localtime:/etc/localtime:ro
     environment:
       - NODE_ENV=production
       - PORT=3000
+      - TZ=Asia/Shanghai
       - JWT_SECRET=${JWT_SECRET:-volunteer-manage-secret-key-2026}