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.

pkg/mail/errs.go

@@ -0,0 +1,16 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package mail 提供 SMTP 邮件发送功能。
+package mail
+
+const (
+	errDialTLSFailed            = "dial tls failed: %w"
+	errSMTPClientCreationFailed = "smtp client creation failed: %w"
+	errSMTPAuthFailed           = "smtp auth failed: %w"
+	errSMTPMailCommandFailed    = "smtp mail command failed: %w"
+	errSMTPRcptCommandFailed    = "smtp rcpt command failed: %w"
+	errSMTPDataCommandFailed    = "smtp data command failed: %w"
+	errSMTPWritingBodyFailed    = "smtp writing body failed: %w" //nolint:gosec // false positive: this is an error message, not hardcoded credentials
+	errSendMailFailed           = "send mail failed: %w"
+)