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/lib/amap.ts

@@ -0,0 +1,7 @@
+import { ApiError } from "@/lib/api";
+
+export function requireAmapApiKey(): string {
+  const key = process.env.AMAP_API_KEY;
+  if (!key) throw new ApiError("服务配置异常，请稍后重试", 500);
+  return key;
+}

src/lib/validation.ts

@@ -0,0 +1,47 @@
+import { ApiError } from "@/lib/api";
+
+export function validateUsername(raw: string): string {
+  const trimmed = raw.trim();
+  if (trimmed.length < 2 || trimmed.length > 16) {
+    throw new ApiError("用户名需要 2-16 个字符");
+  }
+  return trimmed;
+}
+
+export function validatePassword(password: string, label = "密码"): void {
+  if (password.length < 6) {
+    throw new ApiError(`${label}至少 6 个字符`);
+  }
+}
+
+export function validateEmail(email: string): void {
+  if (!/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)) {
+    throw new ApiError("邮箱格式不正确");
+  }
+}
+
+export function validateIdeaContent(raw: unknown): string {
+  if (!raw || typeof raw !== "string" || raw.trim().length === 0) {
+    throw new ApiError("内容不能为空");
+  }
+  const trimmed = raw.trim();
+  if (trimmed.length > 200) {
+    throw new ApiError("内容不能超过 200 字");
+  }
+  return trimmed;
+}
+
+export function validateRoomName(raw: unknown, fallback = "我们的周末"): string {
+  const trimmed = ((raw as string) || "").trim() || fallback;
+  if (trimmed.length > 30) {
+    throw new ApiError("房间名不能超过 30 个字");
+  }
+  return trimmed;
+}
+
+export function requireString(value: unknown, fieldName: string): string {
+  if (!value || typeof value !== "string") {
+    throw new ApiError(`${fieldName}不能为空`);
+  }
+  return value;
+}