refactor(repo): consolidate openflare-server to root and move subprojects to internal/apps

- Merge all files inside openflare-server to the repository root directory.
- Relocate agent, relay, and flared subprojects from internal/ to internal/apps/.
- Combine docker-compose files and update build context paths to root.
- Update GitHub workflows and Dockerfiles to refer to new directories and package names.
- Rewrite Go package imports across all files.
- Resolve database renew test race condition and clean up docs.

pkg/trace/sampler.go

@@ -0,0 +1,19 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package trace
+
+import (
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+)
+
+// ParentBasedRatioSampler 创建父级感知的概率采样器
+// - 如果父 Span 已采样，则子 Span 也采样
+// - 如果父 Span 未采样，则子 Span 也不采样
+// - 如果是根 Span，按 samplingRate 概率采样
+func ParentBasedRatioSampler(samplingRate float64) sdktrace.Sampler {
+	return sdktrace.ParentBased(
+		sdktrace.TraceIDRatioBased(samplingRate),
+	)
+}