refactor: 引入 apiHandler + ApiError，消除 20 个路由的 try/catch 样板

- 新增 src/lib/api.ts：ApiError 错误类 + apiHandler 统一包装器
- 20 个 API 路由统一使用 apiHandler，删除重复的 try/catch 块
- 验证错误改用 throw new ApiError()，减少嵌套层级
- join/manage 路由的错误码映射改为直接抛出 ApiError
- 删除已无引用的 errorResponse 辅助函数
- 净减 273 行代码

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

@@ -1,27 +1,21 @@
-import { NextRequest, NextResponse } from "next/server";
+import { NextResponse } from "next/server";
 import { prisma } from "@/lib/prisma";
 import bcrypt from "bcryptjs";
+import { apiHandler, ApiError } from "@/lib/api";
 
-export async function POST(req: NextRequest) {
+export const POST = apiHandler(async (req) => {
   const { username, password, avatar } = await req.json();
 
-  if (!username || !password) {
-    return NextResponse.json({ error: "用户名和密码为必填项" }, { status: 400 });
-  }
+  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) {
-    return NextResponse.json({ error: "用户名需要 2-16 个字符" }, { status: 400 });
-  }
-
-  if (password.length < 6) {
-    return NextResponse.json({ error: "密码至少 6 个字符" }, { status: 400 });
+    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 existing = await prisma.user.findUnique({ where: { username: trimmedUsername } });
-  if (existing) {
-    return NextResponse.json({ error: "用户名已被注册" }, { status: 409 });
-  }
+  if (existing) throw new ApiError("用户名已被注册", 409);
 
   const passwordHash = await bcrypt.hash(password, 10);
 
@@ -38,4 +32,4 @@ export async function POST(req: NextRequest) {
     username: user.username,
     avatar: user.avatar,
   });
-}
+});