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/db.js

@@ -40,6 +40,9 @@ db.exec(`
   CREATE INDEX IF NOT EXISTS idx_records_boss ON records(boss);
   CREATE INDEX IF NOT EXISTS idx_records_partner ON records(partner);
   CREATE INDEX IF NOT EXISTS idx_records_source ON records(source);
+  CREATE INDEX IF NOT EXISTS idx_records_user_date ON records(user_id, created_at);
+  CREATE INDEX IF NOT EXISTS idx_records_user_category ON records(user_id, category, created_at);
+  CREATE INDEX IF NOT EXISTS idx_records_destination ON records(destination);
 `);
 
 // 迁移：去掉 category 的 CHECK 约束