refactor: 提取分页魔法数字为 pagination 常量

- handler 层: device/log/webhook/user handler 使用 pagination.DefaultPageSize/MaxPageSize
- service 层: device/login_log/operation_log service 使用 pagination.DefaultPageSize
- repository 层: user repository 使用 pagination.DefaultPageSize/MaxPageSize
- 消除 8 处硬编码的 20/100 分页魔法数字

internal/api/handler/user_handler.go

@@ -10,6 +10,7 @@ import (
 	"github.com/user-management-system/internal/api/middleware"
 	"github.com/user-management-system/internal/auth"
 	"github.com/user-management-system/internal/domain"
+	"github.com/user-management-system/internal/pagination"
 	"github.com/user-management-system/internal/service"
 )
 
@@ -115,7 +116,7 @@ func (h *UserHandler) ListUsers(c *gin.Context) {
 
 	// Fallback to legacy offset-based pagination
 	offset, _ := strconv.ParseInt(c.DefaultQuery("offset", "0"), 10, 64)
-	limit, _ := strconv.ParseInt(c.DefaultQuery("limit", "20"), 10, 64)
+	limit, _ := strconv.ParseInt(c.DefaultQuery("limit", strconv.Itoa(pagination.DefaultPageSize)), 10, 64)
 
 	users, total, err := h.userService.List(c.Request.Context(), int(offset), int(limit))
 	if err != nil {