feat: add Reverb realtime messaging

app/Policies/MessagePolicy.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Policies;
+
+use App\Models\ConversationParticipant;
+use App\Models\Message;
+use App\Models\User;
+
+class MessagePolicy
+{
+    public function view(User $user, Message $message): bool
+    {
+        return ConversationParticipant::query()
+            ->where('conversation_id', $message->conversation_id)
+            ->where('user_id', $user->id)
+            ->whereNull('left_at')
+            ->exists();
+    }
+
+    public function update(User $user, Message $message): bool
+    {
+        return $message->sender_id === $user->id && $message->deleted_at === null;
+    }
+
+    public function delete(User $user, Message $message): bool
+    {
+        return $message->sender_id === $user->id || $user->isAdmin();
+    }
+}