refactor(messaging): replace WebSocket with SSE for real-time message handling

Replace WebSocket-based real-time communication with Server-Sent Events (SSE) across the messaging infrastructure. This includes:

- Create new SSEClient datasource to manage SSE connections
- Create new SSEMessageHandler to process SSE events
- Update ProcessMessageUseCase to use SSEClient instead of WebSocketClient
- Update MessageManager and MessageStateManager to work with SSE handlers
- Rename connection state variables from `isWebSocketConnected` to `isSSEConnected`
- Update type definitions in dto.ts (WSEventType → SSEEventType, etc.)
- Delete obsolete WebSocketClient and WebSocketMessageHandler files
- Update comments and documentation to reflect SSE terminology

This refactoring aligns with the backend's SSE implementation for better compatibility with React Native's networking capabilities.

src/services/backgroundService.ts

@@ -4,7 +4,7 @@
  * 
  * 功能：
  * - 定期后台任务保持 App 在内存中
- * - 配合 WebSocket 服务保持长连接
+ * - 配合 SSE 服务保持长连接
  * - 收到消息时触发震动提示
  */
 
@@ -47,7 +47,7 @@ let appStateSubscription: any = null;
 // 定义后台任务
 TaskManager.defineTask(BACKGROUND_FETCH_TASK, async () => {
   try {
-    // 检查 WebSocket 连接状态
+    // 检查 SSE 连接状态
     if (!sseService.isConnected()) {
       await sseService.connect();
     }
@@ -60,7 +60,7 @@ TaskManager.defineTask(BACKGROUND_FETCH_TASK, async () => {
   }
 });
 
-// WebSocket 保活任务
+// SSE 保活任务
 TaskManager.defineTask(REALTIME_KEEPALIVE_TASK, async () => {
   try {
     if (!sseService.isConnected()) {
@@ -174,9 +174,9 @@ async function registerBackgroundTasks(): Promise<void> {
       });
     }
 
-    // 注册 WebSocket 保活任务
-    const isWsKeepaliveRegistered = await TaskManager.isTaskRegisteredAsync(REALTIME_KEEPALIVE_TASK);
-    if (!isWsKeepaliveRegistered) {
+    // 注册 SSE 保活任务
+    const isSseKeepaliveRegistered = await TaskManager.isTaskRegisteredAsync(REALTIME_KEEPALIVE_TASK);
+    if (!isSseKeepaliveRegistered) {
       await BackgroundFetch.registerTaskAsync(REALTIME_KEEPALIVE_TASK, {
         minimumInterval: 60, // 1 分钟检查一次
         stopOnTerminate: false,