refactor：拆分 API 路由与请求校验，统一 final 类和代码风格

routes/api/v1/admin/auth.php

@@ -0,0 +1,16 @@
+<?php
+
+use Illuminate\Support\Facades\Route;
+use App\Http\Controllers\Api\V1\Admin\Auth\LoginController;
+use App\Http\Controllers\Api\V1\Admin\Auth\CaptchaController;
+
+/**
+ * 管理员认证路由（无需 Token，需验证码/限流）。
+ */
+Route::middleware('throttle:admin-auth-captcha')
+    ->get('auth/captcha', CaptchaController::class)
+    ->name('api.v1.admin.auth.captcha');
+
+Route::middleware('throttle:admin-auth-login')
+    ->post('auth/login', LoginController::class)
+    ->name('api.v1.admin.auth.login');