perf(packages): 优化 /api/packages 7000+ 条响应性能

- buildPackages 缓存全量结果（rawCache/ignored/threshold 不变时复用）

- gzip 压缩大响应（Node 内置 zlib，减少传输体积 60-80%）

backend/server.js

@@ -20,6 +20,7 @@ process.env.UV_THREADPOOL_SIZE = process.env.UV_THREADPOOL_SIZE || '8'
 import express from 'express'
 import path from 'node:path'
 import fs from 'node:fs'
+import zlib from 'node:zlib'
 import { fileURLToPath } from 'node:url'
 import { resolveStorageDir } from './scanner.js'
 import { isDemoMode, scanState, runScanAsync } from './scan-manager.js'
@@ -40,6 +41,26 @@ const __dirname = path.dirname(fileURLToPath(import.meta.url))
 const app = express()
 app.use(express.json({ limit: '1mb' }))
 
+// gzip 压缩：大响应（如 /api/packages 7000+ 条）减少传输体积 60-80%，Node 内置 zlib 无需新依赖
+app.use((req, res, next) => {
+  const accept = req.headers['accept-encoding'] || ''
+  if (!accept.includes('gzip')) return next()
+  const originalJson = res.json.bind(res)
+  res.json = (data) => {
+    try {
+      const json = JSON.stringify(data)
+      if (json.length < 1024) return originalJson(data) // 小响应不压缩，避免开销
+      const compressed = zlib.gzipSync(json)
+      res.setHeader('Content-Encoding', 'gzip')
+      res.setHeader('Content-Type', 'application/json')
+      res.send(compressed)
+    } catch {
+      originalJson(data)
+    }
+  }
+  next()
+})
+
 const PORT = Number(process.env.PORT || 3000)
 
 // —— 路由装配：登录/登出/主题公开，其余 /api/* 需鉴权 ——