feat(post): add idempotency support for post creation with client request ID

Add client_request_id field to post creation endpoints (CreateVotePost and CreatePostWithSegments).
When provided, the server uses Redis SetNX-based idempotency to prevent duplicate posts
from double-clicks or network retries within a 24-hour TTL. Duplicate requests return
the originally created post; concurrent in-flight requests return 429 "正在发布中，请稍候".

internal/handler/vote_handler.go

@@ -47,6 +47,11 @@ func (h *VoteHandler) CreateVotePost(c *gin.Context) {
 			response.BadRequest(c, moderationErr.UserMessage())
 			return
 		}
+		// 幂等命中"进行中"占位：提示客户端稍候，避免重复提交
+		if errors.Is(err, service.ErrDuplicatePostRequest) {
+			response.ErrorWithStatusCode(c, http.StatusTooManyRequests, 429, err.Error())
+			return
+		}
 		response.Error(c, http.StatusBadRequest, err.Error())
 		return
 	}