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/openflare/flared/errs.go

@@ -0,0 +1,9 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package flared
+
+const (
+	errTunnelTokenInvalid     = "无权进行此操作，Tunnel Token 无效"
+	errTunnelNodeTypeMismatch = "此节点不是 TunnelClient 类型"
+)