feat: 即時預約聊天室（realtime booking chat）

## 新增功能
- 會員與教練在 confirmed 預約期間可互傳文字與圖片訊息
- Presence Channel 顯示對方在線狀態（即時加入/離線）
- 已讀回執：對方讀取訊息後即時更新
- 預約完成（completed）後訊息封存唯讀
- 圖片上傳使用 intervention/image 處理（移除 EXIF、限制尺寸、強制重新編碼）

## 通知系統
- 收到新訊息時 Bell Icon 即時更新（NotificationCreated via private channel）
- 預約列表卡片顯示未讀角標（GET /api/bookings/messages/unread-counts 批次查詢）
- 瀏覽器分頁在背景時推送 Web Notification

## 基礎建設
- 引入 Laravel Reverb 作為自架 WebSocket 伺服器
- docker-compose 新增 reverb service（連接 proxy_net，供 NPM 代理）
- 前端安裝 laravel-echo + pusher-js，初始化 Echo plugin
- Dockerfile 補 GD JPEG/WebP/FreeType 支援

## 清理
- 移除 test_broadcast.php 與 resources/js/echo.js（Blade 殘留）
- 移除 /ping、/testpost 測試路由
- channels.php 改用 class-based 授權語法，移除 debug log
- BookingChat.vue otherType 提取為 computed 消除重複
- docker-entrypoint.sh 健康檢查改用 env var 取代硬編碼密碼

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

app/Events/MessageSent.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace App\Events;
+
+use App\Models\BookingMessage;
+use Illuminate\Broadcasting\InteractsWithSockets;
+use Illuminate\Broadcasting\PresenceChannel;
+use Illuminate\Contracts\Broadcasting\ShouldBroadcastNow;
+use Illuminate\Foundation\Events\Dispatchable;
+use Illuminate\Queue\SerializesModels;
+
+class MessageSent implements ShouldBroadcastNow
+{
+    use Dispatchable, InteractsWithSockets, SerializesModels;
+
+    public function __construct(public BookingMessage $message) {}
+
+    public function broadcastAs(): string
+    {
+        return 'MessageSent';
+    }
+
+    public function broadcastOn(): array
+    {
+        return [
+            new PresenceChannel('booking.' . $this->message->booking_id),
+        ];
+    }
+
+    public function broadcastWith(): array
+    {
+        return [
+            'id'          => $this->message->id,
+            'sender_id'   => $this->message->sender_id,
+            'sender_type' => $this->message->sender_type,
+            'type'        => $this->message->type,
+            'content'     => $this->message->content,
+            'created_at'  => $this->message->created_at->toISOString(),
+        ];
+    }
+}