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/task/errs.go

@@ -0,0 +1,17 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package task
+
+const (
+	errUnknownTaskType            = "未知的任务类型: %s"
+	errCreateTaskExecutionFailed  = "创建任务执行记录失败: %w"
+	errTaskEnqueueFailed          = "任务入队失败: %w"
+	errTaskExecutionNotFound      = "任务执行记录不存在: %w"
+	errRetryOnlyFailedTask        = "只有失败的任务才能重试，当前状态: %s"
+	errTaskNotRetryable           = "该任务不支持重试"
+	errTaskMaxRetryExceeded       = "已达到最大重试次数 %d"
+	errCreateRetryExecutionFailed = "创建重试任务执行记录失败: %w"
+	errRetryTaskEnqueueFailed     = "重试任务入队失败: %w"
+	errUnregisteredTaskHandler    = "未注册的任务处理器: %s"
+)