feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/tests/helpers.go

@@ -0,0 +1,74 @@
+package tests
+
+import (
+	"fmt"
+	"testing"
+
+	"nex/backend/internal/config"
+
+	"github.com/stretchr/testify/assert"
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+)
+
+// SetupTestDB initializes a temporary SQLite database with auto-migration.
+func SetupTestDB(t *testing.T) *gorm.DB {
+	t.Helper()
+
+	dir := t.TempDir()
+	dsn := dir + "/test.db"
+
+	db, err := gorm.Open(sqlite.Open(dsn), &gorm.Config{})
+	assert.NoError(t, err, "failed to open test database")
+
+	err = db.AutoMigrate(&config.Provider{}, &config.Model{}, &config.UsageStats{})
+	assert.NoError(t, err, "failed to auto-migrate test database")
+
+	return db
+}
+
+// CleanupTestDB closes the database and removes the temp database file.
+func CleanupTestDB(t *testing.T, db *gorm.DB) {
+	t.Helper()
+
+	sqlDB, err := db.DB()
+	assert.NoError(t, err, "failed to get underlying sql.DB")
+
+	err = sqlDB.Close()
+	assert.NoError(t, err, "failed to close test database")
+}
+
+// CreateTestProvider creates a test provider and returns it.
+func CreateTestProvider(t *testing.T, db *gorm.DB, id string) config.Provider {
+	t.Helper()
+
+	provider := config.Provider{
+		ID:      id,
+		Name:    fmt.Sprintf("test-provider-%s", id),
+		APIKey:  fmt.Sprintf("test-api-key-%s", id),
+		BaseURL: fmt.Sprintf("https://api.test-%s.com", id),
+		Enabled: true,
+	}
+
+	err := db.Create(&provider).Error
+	assert.NoError(t, err, "failed to create test provider")
+
+	return provider
+}
+
+// CreateTestModel creates a test model and returns it.
+func CreateTestModel(t *testing.T, db *gorm.DB, id string, providerID string, modelName string) config.Model {
+	t.Helper()
+
+	model := config.Model{
+		ID:         id,
+		ProviderID: providerID,
+		ModelName:  modelName,
+		Enabled:    true,
+	}
+
+	err := db.Create(&model).Error
+	assert.NoError(t, err, "failed to create test model")
+
+	return model
+}