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/agent/errs.go

@@ -0,0 +1,20 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package agent
+
+const (
+	errMissingAgentToken      = "缺少 Agent Token"
+	errInvalidAgentToken      = "无权进行此操作，Agent Token 无效"
+	errInvalidDiscoveryToken  = "无权进行此操作，注册 Token 无效"
+	errNodeMissingFromContext = "Node object missing from context"
+	errNoActiveConfig         = "当前没有激活版本"
+	errNodeNotFound           = "节点不存在"
+	errNodeIDRequired         = "node_id 不能为空"
+	errVersionRequired        = "version 不能为空"
+	errInvalidApplyResult     = "result 仅支持 success、warning 或 failed"
+	errIPRequired             = "ip 不能为空"
+	errIPInvalid              = "ip 格式无效"
+	errAgentVersionRequired   = "version 不能为空"
+	errNodeIDConflict         = "节点标识生成冲突，请重试"
+)