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/pkg/response/abort.go

@@ -0,0 +1,45 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package response
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// AbortBadRequest 以 400 中断请求并将错误挂载到 Gin Error 链，供全局中间件统一记录 Trace 并响应。
+func AbortBadRequest(c *gin.Context, msg string) {
+	AbortWithError(c, http.StatusBadRequest, msg)
+}
+
+// AbortUnauthorized 以 401 中断请求并将错误挂载到 Gin Error 链。
+func AbortUnauthorized(c *gin.Context, msg string) {
+	AbortWithError(c, http.StatusUnauthorized, msg)
+}
+
+// AbortForbidden 以 403 中断请求并将错误挂载到 Gin Error 链。
+func AbortForbidden(c *gin.Context, msg string) {
+	AbortWithError(c, http.StatusForbidden, msg)
+}
+
+// AbortNotFound 以 404 中断请求并将错误挂载到 Gin Error 链。
+func AbortNotFound(c *gin.Context, msg string) {
+	AbortWithError(c, http.StatusNotFound, msg)
+}
+
+// AbortInternal 以 500 中断请求并将错误挂载到 Gin Error 链。
+func AbortInternal(c *gin.Context, msg string) {
+	AbortWithError(c, http.StatusInternalServerError, msg)
+}
+
+// AbortTooManyRequests 以 429 中断请求并将错误挂载到 Gin Error 链。
+func AbortTooManyRequests(c *gin.Context, msg string) {
+	AbortWithError(c, http.StatusTooManyRequests, msg)
+}
+
+// AbortConflict 以 409 中断请求并将错误挂载到 Gin Error 链。
+func AbortConflict(c *gin.Context, msg string) {
+	AbortWithError(c, http.StatusConflict, msg)
+}