refactor: 提取 validation.ts 和 amap.ts，统一 API 路由校验逻辑

新增 validation.ts（用户名/密码/邮箱/内容/房间名/必填字段校验）
和 amap.ts（AMAP API key 校验），消除 7 个路由中的重复验证代码。

src/app/api/auth/register/route.ts

@@ -2,17 +2,15 @@ import { NextResponse } from "next/server";
 import { prisma } from "@/lib/prisma";
 import bcrypt from "bcryptjs";
 import { apiHandler, ApiError } from "@/lib/api";
+import { validateUsername, validatePassword } from "@/lib/validation";
 
 export const POST = apiHandler(async (req) => {
   const { username, password, avatar } = await req.json();
 
   if (!username || !password) throw new ApiError("用户名和密码为必填项");
 
-  const trimmedUsername = username.trim();
-  if (trimmedUsername.length < 2 || trimmedUsername.length > 16) {
-    throw new ApiError("用户名需要 2-16 个字符");
-  }
-  if (password.length < 6) throw new ApiError("密码至少 6 个字符");
+  const trimmedUsername = validateUsername(username);
+  validatePassword(password);
 
   const existing = await prisma.user.findUnique({ where: { username: trimmedUsername } });
   if (existing) throw new ApiError("用户名已被注册", 409);