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/lib/api.ts

@@ -0,0 +1,36 @@
+import { NextRequest, NextResponse } from "next/server";
+
+export class ApiError extends Error {
+  constructor(
+    message: string,
+    public status: number = 400,
+  ) {
+    super(message);
+  }
+}
+
+type RouteContext = { params: Promise<Record<string, string>> };
+
+type RouteHandler = (
+  req: NextRequest,
+  ctx: RouteContext,
+) => Promise<NextResponse>;
+
+/**
+ * Wraps a Next.js route handler with unified error handling.
+ * - ApiError instances are converted to JSON responses with matching status codes
+ * - Unknown errors are logged and returned as 500
+ */
+export function apiHandler(handler: RouteHandler): RouteHandler {
+  return async (req, ctx) => {
+    try {
+      return await handler(req, ctx);
+    } catch (e) {
+      if (e instanceof ApiError) {
+        return NextResponse.json({ error: e.message }, { status: e.status });
+      }
+      console.error(`[API ${req.method} ${req.nextUrl.pathname}]`, e);
+      return NextResponse.json({ error: "操作失败" }, { status: 500 });
+    }
+  };
+}