feat: notificaciones en tiempo real para social (likes, comentarios, solicitudes)

- Notifier helper: guarda en BD + broadcast en un solo llamado
- AppNotification: notificación genérica para canal database
- FriendshipController: notifica al recibir solicitud y al aceptarla
- PostController: notifica al autor cuando le dan like o comentan (excepto en propio post)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Notifications/AppNotification.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace App\Notifications;
+
+use Illuminate\Bus\Queueable;
+use Illuminate\Notifications\Notification;
+
+class AppNotification extends Notification
+{
+    use Queueable;
+
+    public function __construct(
+        public readonly string  $title,
+        public readonly string  $body  = '',
+        public readonly ?string $url   = null,
+        public readonly string  $icon  = 'fa-bell',
+        public readonly string  $color = 'primary',
+    ) {}
+
+    public function via(object $notifiable): array
+    {
+        return ['database'];
+    }
+
+    public function toArray(object $notifiable): array
+    {
+        return [
+            'title' => $this->title,
+            'body'  => $this->body,
+            'url'   => $this->url,
+            'icon'  => $this->icon,
+            'color' => $this->color,
+        ];
+    }
+}