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/bootstrap/app.php

@@ -12,7 +12,9 @@
         health: '/up',
     )
     ->withMiddleware(function (Middleware $middleware) {
-        //
+        $middleware->alias([
+            'registration.enabled' => \App\Http\Middleware\CheckRegistrationEnabled::class,
+        ]);
     })
     ->withExceptions(function (Exceptions $exceptions) {
         //