refactor: 系统优化 - 错误处理、输入验证、无限滚动

主要改进:
- 新增配置管理 (server/config.js)
- 新增日志工具 (server/utils/logger.js)
- 添加全局错误处理中间件，统一错误响应格式
- 添加输入验证和 AppError 错误类
- 重构 db.js 迁移逻辑，代码更清晰
- 前端列表改为无限滚动加载，每页 20 条
- 封装 AppState 状态管理模块

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

server/utils/logger.js

@@ -0,0 +1,79 @@
+/**
+ * 简单日志工具
+ * 支持日志级别和格式化输出
+ */
+
+const config = require('../config');
+
+const LEVELS = {
+  debug: 0,
+  info: 1,
+  warn: 2,
+  error: 3
+};
+
+const COLORS = {
+  debug: '\x1b[36m',   // cyan
+  info: '\x1b[32m',    // green
+  warn: '\x1b[33m',    // yellow
+  error: '\x1b[31m',   // red
+  reset: '\x1b[0m'
+};
+
+const currentLevel = LEVELS[config.log.level] || LEVELS.info;
+const colorize = config.log.colorize && process.stdout.isTTY;
+
+function formatTime() {
+  const d = new Date();
+  return d.toISOString().replace('T', ' ').slice(0, 19);
+}
+
+function formatMessage(level, ...args) {
+  const timestamp = formatTime();
+  const prefix = `[${timestamp}] [${level.toUpperCase()}]`;
+
+  if (colorize) {
+    return `${COLORS[level]}${prefix}${COLORS.reset} ${args.join(' ')}`;
+  }
+  return `${prefix} ${args.join(' ')}`;
+}
+
+const logger = {
+  debug(...args) {
+    if (currentLevel <= LEVELS.debug) {
+      console.log(formatMessage('debug', ...args));
+    }
+  },
+
+  info(...args) {
+    if (currentLevel <= LEVELS.info) {
+      console.log(formatMessage('info', ...args));
+    }
+  },
+
+  warn(...args) {
+    if (currentLevel <= LEVELS.warn) {
+      console.warn(formatMessage('warn', ...args));
+    }
+  },
+
+  error(...args) {
+    if (currentLevel <= LEVELS.error) {
+      console.error(formatMessage('error', ...args));
+    }
+  },
+
+  // 请求日志中间件
+  requestLogger(req, res, next) {
+    const start = Date.now();
+    res.on('finish', () => {
+      const duration = Date.now() - start;
+      const status = res.statusCode;
+      const level = status >= 400 ? 'warn' : 'info';
+      logger[level](`${req.method} ${req.path} ${status} ${duration}ms`);
+    });
+    next();
+  }
+};
+
+module.exports = logger;