perf: 系统性能优化 - 数据库索引、缓存、GZIP压缩和查询合并

- 添加3个复合索引提升查询性能 (user_date, user_category, destination)
- 添加GZIP压缩和静态资源缓存 (maxAge: 1h)
- 添加API响应no-cache头
- 添加stats端点2分钟内存缓存 (per-user key, LRU淘汰)
- 合并多次数据库查询为单次查询
- 添加滚动防抖改善移动端体验
- 修复res.once内存泄漏问题

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

server/app.js

@@ -1,6 +1,7 @@
 const express = require('express');
 const cors = require('cors');
 const path = require('path');
+const compression = require('compression');
 
 const db = require('./db');
 const { router: authRouter, auth } = require('./routes/auth');
@@ -12,9 +13,18 @@ const app = express();
 
 app.use(cors());
 app.use(express.json());
-app.use(express.static(path.join(__dirname, '..', 'public')));
+app.use(compression());
+app.use(express.static(path.join(__dirname, '..', 'public'), {
+  maxAge: '1h',
+  etag: true
+}));
 
 // 联系人自动补全
+app.use('/api', (req, res, next) => {
+  res.setHeader('Cache-Control', 'no-cache');
+  next();
+});
+
 app.get('/api/contacts', auth, (req, res) => {
   const field = req.query.field;
   const allowed = ['boss', 'partner', 'source', 'destination'];