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/util/errs.go

@@ -0,0 +1,10 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+const (
+	errCreateHTTPRequestFailed = "创建HTTP请求失败: %w"
+	errHTTPRequestFailed       = "请求%s接口失败: %w"
+	errInvalidCustomValue      = "invalid value: %v"
+)