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/trace/trace_provider.go

@@ -0,0 +1,52 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package trace
+
+import (
+	"context"
+	"os"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
+	"go.opentelemetry.io/otel/sdk/resource"
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+)
+
+func newTracerProvider(cfg Config) (*sdktrace.TracerProvider, error) {
+	// 获取主机名和容器信息
+	hostname, err := os.Hostname()
+	if err != nil {
+		return nil, err
+	}
+
+	// 业务属性不绑定 schema URL，合并时继承 resource.Default() 的 SDK 内置版本，避免 semconv 与 otel/sdk 升级不同步。
+	r, err := resource.Merge(
+		resource.Default(),
+		resource.NewSchemaless(
+			attribute.String("service.name", cfg.AppName),
+			attribute.String("host.name", hostname),
+			attribute.String("k8s.namespace.name", os.Getenv("KUBERNETES_NAMESPACE")),
+			attribute.String("k8s.pod.name", os.Getenv("KUBERNETES_POD_NAME")),
+			attribute.String("k8s.pod.uid", os.Getenv("KUBERNETES_POD_UID")),
+		),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	// 初始化 Exporter
+	traceExporter, err := otlptracegrpc.New(context.Background())
+	if err != nil {
+		return nil, err
+	}
+
+	// 初始化 Trace
+	tracerProvider := sdktrace.NewTracerProvider(
+		sdktrace.WithBatcher(traceExporter),
+		sdktrace.WithResource(r),
+		sdktrace.WithSampler(ParentBasedRatioSampler(cfg.SamplingRate)),
+	)
+	return tracerProvider, nil
+}