feat: add API key authentication middleware

Adds `VerifyApiKey` middleware to validate `X-Api-Key` header against `app.api_secret_key` config. Returns 403 if key is missing or invalid.

app/Http/Middleware/VerifyApiKey.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Http\Middleware;
+
+use Closure;
+use Illuminate\Http\Request;
+use Symfony\Component\HttpFoundation\Response;
+
+class VerifyApiKey
+{
+    /**
+     * Handle an incoming request.
+     *
+     * @param  Closure(Request): (Response)  $next
+     */
+    public function handle(Request $request, Closure $next): Response
+    {
+        if ($request->header('X-Api-Key') !== config('app.api_secret_key')) {
+            abort(403);
+        }
+
+        return $next($request);
+    }
+}