feat(chat): add endpoint to get quoted message by ID

Add GET /api/v1/messages/:id endpoint to retrieve quoted/replied messages
for preview fill and navigation purposes. The endpoint includes authorization
checks ensuring only conversation participants can access the message.

Additionally improve file upload handling for native clients (expo-file-system)
by accepting explicit displayName from frontend, with sanitization to prevent
path traversal and 255-character length limit. File extension detection now
prioritizes the real display name over the multipart header filename.

internal/router/router.go

@@ -407,6 +407,8 @@ func (r *Router) setupRoutes() {
 		messages := v1.Group("/messages")
 		messages.Use(authMiddleware)
 		{
+			// 按 ID 获取被引用消息（引用预览回填/跳转）
+			messages.GET("/:id", r.MessageHandler.GetReplyMessage)
 			// 撤回/删除消息（统一接口）
 			messages.POST("/delete_msg", middleware.RequireVerified(r.UserService), r.MessageHandler.HandleDeleteMsg)
 		}