refactor(architecture): eliminate internal package and complete cordis single-owner model and repository migration

- Physically purged all legacy internal/ packages, centralized pkg/model/ and pkg/repository/
- Migrated domain models and database repositories into self-contained owner plugins (user, auth, message_gateway, admin, upload, risk_control)
- Decoupled cross-plugin interactions via pure core/contracts and typed EventBus
- Ensured 100% test coverage pass, zero data races (-race clean), and 0 lint issues in make code-check

pkg/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"
+)