refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/admin/middlewares.go

@@ -0,0 +1,44 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package admin
+
+import (
+	"github.com/Rain-kl/Wavelet/backend/core/contracts"
+	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
+	"github.com/Rain-kl/Wavelet/backend/pkg/response"
+	"github.com/Rain-kl/Wavelet/backend/pkg/trace"
+	"github.com/Rain-kl/Wavelet/backend/pkg/util"
+	"github.com/gin-gonic/gin"
+)
+
+// LoginAdminRequired 返回管理员权限校验中间件
+func LoginAdminRequired() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		ctx, span := trace.Start(c.Request.Context(), "LoginAdminRequired")
+		defer span.End()
+
+		user, _ := util.GetFromContext[*contracts.UserDTO](c, contracts.AuthUserObjKey)
+		if user == nil {
+			response.AbortNotFound(c, AdminRequired)
+			return
+		}
+
+		// 如果是通过 Access Token 鉴权，需要检查令牌本身是否具有管理员权限
+		if tokenAuth, _ := util.GetFromContext[bool](c, contracts.AuthTokenAuthKey); tokenAuth {
+			tokenAdmin, _ := util.GetFromContext[bool](c, contracts.AuthTokenAdminKey)
+			if !tokenAdmin {
+				response.AbortNotFound(c, TokenAdminRequired)
+				return
+			}
+		}
+
+		if !user.IsAdmin {
+			response.AbortNotFound(c, AdminRequired)
+			return
+		}
+
+		logger.InfoF(ctx, "[LoginAdminRequired] %d %s", user.ID, user.Username)
+		c.Next()
+	}
+}