refactor(structure): group platform, infra, and shared packages

Reorganize internal packages into platform/infra/shared layers and update
imports, docs, and seed-count tests to match current system configs.

internal/shared/errs.go

@@ -0,0 +1,11 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package shared
+
+// 通用业务错误消息常量
+const (
+	BannedAccount = "账号已被封禁"
+	UnAuthorized  = "未登录"
+)