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.

internal/apps/admin/system_config/errs.go

@@ -0,0 +1,15 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package system_config 提供系统配置管理功能
+package system_config
+
+// 系统配置错误消息常量
+const (
+	SystemConfigNotFound                 = "系统配置不存在"
+	ConfigKeyRequired                    = "配置键不能为空"
+	ConfigValueRequired                  = "配置值不能为空"
+	ConfigKeyExists                      = "配置键已存在"
+	StorageDriverSwitchRequiresMigration = "存在存量文件，请通过存储迁移任务切换存储引擎"
+)