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/handler/upload_handler.go

@@ -109,8 +109,12 @@ func (h *UploadHandler) UploadFile(c *gin.Context) {
 	}
 
 	folder := c.DefaultPostForm("folder", "chat")
+	// 前端（尤其原生端 expo-file-system）无法自定义 multipart filename，
+	// 文件常被复制到缓存目录并以 UUID 命名，导致 header Filename 丢失真实名。
+	// 因此前端额外把真实文件名通过 "name" 字段回传，作为权威展示名。
+	displayName := c.PostForm("name")
 
-	result, err := h.uploadService.UploadFile(c.Request.Context(), file, folder, userID)
+	result, err := h.uploadService.UploadFile(c.Request.Context(), file, folder, userID, displayName)
 	if err != nil {
 		// 参数类错误（大小/类型）返回 400，其余返回 500
 		if file.Size > 0 && file.Size <= 100<<20 {