Redirect landing to login, add registration toggle

Changes:
- Landing page (/) now redirects to /login
- Added 'Enable User Registration' setting (off by default)
- Created CheckRegistrationEnabled middleware
- Registration routes blocked when setting is disabled
- Admin can toggle registration in Settings > System

src/app/Http/Middleware/CheckRegistrationEnabled.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Http\Middleware;
+
+use App\Models\Setting;
+use Closure;
+use Illuminate\Http\Request;
+use Symfony\Component\HttpFoundation\Response;
+
+class CheckRegistrationEnabled
+{
+    /**
+     * Handle an incoming request.
+     * Block registration routes if registration is disabled in settings.
+     */
+    public function handle(Request $request, Closure $next): Response
+    {
+        $registrationEnabled = Setting::get('enable_registration', false);
+
+        if (!$registrationEnabled) {
+            abort(403, 'User registration is currently disabled. Please contact an administrator.');
+        }
+
+        return $next($request);
+    }
+}