Fix AdminNotificationService crashing on string throttle_minutes config

env('ERROR_NOTIFICATION_THROTTLE_MINUTES', 15) returns the literal string
"15" from production's .env (env() doesn't cast numeric strings), which
Carbon 3.11's addMinutes() rejects with a TypeError. That crash happened
before the throttle-dedup cache write, so it silently masked the *original*
exception on every notify-worthy error, indefinitely (discovered while
investigating a missing workshop-registration email — the real SMTP error
was hidden behind this crash). Cast to (int).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/Unit/Services/AdminNotificationServiceTest.php

@@ -184,4 +184,27 @@ describe('notifyError', function () {
 
         Notification::assertSentOnDemandTimes(SystemErrorNotification::class, 2);
     });
+
+    it('does not throw and still sends notification when throttle_minutes config is a string', function () {
+        // Reproduces a live production bug: env('ERROR_NOTIFICATION_THROTTLE_MINUTES', 15) returns
+        // the string "15" (env() never casts numeric-looking strings to int), which used to be
+        // passed straight into now()->addMinutes(), and Carbon 3.11 throws a TypeError for a
+        // string argument there. That TypeError masked the original exception being reported and
+        // notifyError() never actually sent the admin notification.
+        Notification::fake();
+        Cache::flush();
+        config(['libra.notifications.throttle_minutes' => '15']);
+
+        $service = new AdminNotificationService;
+        $exception = new TransportException('SMTP error');
+
+        $service->notifyError($exception);
+
+        Notification::assertSentOnDemand(
+            SystemErrorNotification::class,
+            function ($notification, $channels, $notifiable) {
+                return $notifiable->routes['mail'] === 'admin@test.com';
+            }
+        );
+    });
 });