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/middleware.go

@@ -0,0 +1,40 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package response
+
+import (
+	"errors"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"go.opentelemetry.io/otel/codes"
+	"go.opentelemetry.io/otel/trace"
+)
+
+// ErrorHandlerMiddleware 捕获 c.Errors 并统一格式化为 JSON 返回给客户端，同时将其记录到 Span 异常中。
+// 与 AbortWithError / AbortBadRequest 等配合使用，是全局 OTel 友好错误响应的唯一出口。
+func ErrorHandlerMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Next()
+
+		if len(c.Errors) == 0 || c.Writer.Written() {
+			return
+		}
+
+		err := c.Errors.Last().Err
+		span := trace.SpanFromContext(c.Request.Context())
+		if span.IsRecording() {
+			span.RecordError(err)
+			span.SetStatus(codes.Error, err.Error())
+		}
+
+		var apiErr *APIError
+		if errors.As(err, &apiErr) {
+			c.JSON(apiErr.Code, Err(apiErr.Msg))
+			return
+		}
+
+		c.JSON(http.StatusInternalServerError, Err("内部系统错误"))
+	}
+}