feat: 文章属性 — Post Model 扩展 + 置顶系统

- Post 新增 7 字段：Visibility/PostType/ReprintSource/Declaration/ReprintProhibited/PinType/PinnedAt
- 公开列表过滤 private 文章（管理员可见全部）
- 列表排序：全局置顶 > 按时间
- declarationLabel 模板函数（7 种创作声明）
- Pin/Unpin API（admin+，category/global 两种置顶类型）

internal/controller/admin/admin_post_action_controller.go

@@ -103,6 +103,44 @@ func (ctrl *AdminPostController) Unlock(c *gin.Context) {
 	common.OkMessage(c, "已解锁")
 }
 
+// Pin 置顶文章
+func (ctrl *AdminPostController) Pin(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		common.Error(c, http.StatusBadRequest, "无效的帖子 ID")
+		return
+	}
+
+	var req struct {
+		PinType string `json:"pin_type" binding:"required"`
+	}
+	if err := c.ShouldBindJSON(&req); err != nil || (req.PinType != model.PinTypeCategory && req.PinType != model.PinTypeGlobal) {
+		common.Error(c, http.StatusBadRequest, "无效的置顶类型")
+		return
+	}
+
+	if err := ctrl.postService.Pin(uint(id), req.PinType); err != nil {
+		common.Error(c, http.StatusInternalServerError, "置顶失败")
+		return
+	}
+	common.OkMessage(c, "已置顶")
+}
+
+// Unpin 取消置顶
+func (ctrl *AdminPostController) Unpin(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		common.Error(c, http.StatusBadRequest, "无效的帖子 ID")
+		return
+	}
+
+	if err := ctrl.postService.Unpin(uint(id)); err != nil {
+		common.Error(c, http.StatusInternalServerError, "取消置顶失败")
+		return
+	}
+	common.OkMessage(c, "已取消置顶")
+}
+
 // Restore 恢复软删除
 func (ctrl *AdminPostController) Restore(c *gin.Context) {
 	id, err := strconv.ParseUint(c.Param("id"), 10, 64)