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/persistence/idgen/snowflake_test.go

@@ -0,0 +1,15 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package idgen
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNextUint64ID(t *testing.T) {
+	id := NextUint64ID()
+	assert.NotZero(t, id)
+}