feat: add Reverb realtime messaging

app/Policies/ConversationPolicy.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace App\Policies;
+
+use App\Models\Conversation;
+use App\Models\ConversationParticipant;
+use App\Models\User;
+
+class ConversationPolicy
+{
+    public function view(User $user, Conversation $conversation): bool
+    {
+        return $this->participantRecord($user, $conversation) !== null
+            && (bool) ($conversation->is_active ?? true);
+    }
+
+    public function send(User $user, Conversation $conversation): bool
+    {
+        return $this->view($user, $conversation);
+    }
+
+    public function manageParticipants(User $user, Conversation $conversation): bool
+    {
+        $participant = $this->participantRecord($user, $conversation);
+
+        return $participant !== null && $participant->role === 'admin';
+    }
+
+    public function rename(User $user, Conversation $conversation): bool
+    {
+        return $conversation->isGroup() && $this->manageParticipants($user, $conversation);
+    }
+
+    public function joinPresence(User $user, Conversation $conversation): bool
+    {
+        return $this->view($user, $conversation);
+    }
+
+    private function participantRecord(User $user, Conversation $conversation): ?ConversationParticipant
+    {
+        return ConversationParticipant::query()
+            ->where('conversation_id', $conversation->id)
+            ->where('user_id', $user->id)
+            ->whereNull('left_at')
+            ->first();
+    }
+}