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/internal/repository/provider_repo_impl.go

@@ -0,0 +1,94 @@
+package repository
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+
+	"nex/backend/internal/config"
+	"nex/backend/internal/domain"
+	appErrors "nex/backend/pkg/errors"
+)
+
+type providerRepository struct {
+	db *gorm.DB
+}
+
+func NewProviderRepository(db *gorm.DB) ProviderRepository {
+	return &providerRepository{db: db}
+}
+
+func (r *providerRepository) Create(provider *domain.Provider) error {
+	p := toConfigProvider(provider)
+	p.CreatedAt = time.Now()
+	p.UpdatedAt = time.Now()
+	return r.db.Create(&p).Error
+}
+
+func (r *providerRepository) GetByID(id string) (*domain.Provider, error) {
+	var p config.Provider
+	err := r.db.First(&p, "id = ?", id).Error
+	if err != nil {
+		return nil, err
+	}
+	d := toDomainProvider(&p)
+	return &d, nil
+}
+
+func (r *providerRepository) List() ([]domain.Provider, error) {
+	var providers []config.Provider
+	err := r.db.Find(&providers).Error
+	if err != nil {
+		return nil, err
+	}
+	result := make([]domain.Provider, len(providers))
+	for i := range providers {
+		result[i] = toDomainProvider(&providers[i])
+	}
+	return result, nil
+}
+
+func (r *providerRepository) Update(id string, updates map[string]interface{}) error {
+	updates["updated_at"] = time.Now()
+	result := r.db.Model(&config.Provider{}).Where("id = ?", id).Updates(updates)
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return appErrors.ErrProviderNotFound
+	}
+	return nil
+}
+
+func (r *providerRepository) Delete(id string) error {
+	result := r.db.Delete(&config.Provider{}, "id = ?", id)
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return appErrors.ErrProviderNotFound
+	}
+	return nil
+}
+
+func toDomainProvider(p *config.Provider) domain.Provider {
+	return domain.Provider{
+		ID:        p.ID,
+		Name:      p.Name,
+		APIKey:    p.APIKey,
+		BaseURL:   p.BaseURL,
+		Enabled:   p.Enabled,
+		CreatedAt: p.CreatedAt,
+		UpdatedAt: p.UpdatedAt,
+	}
+}
+
+func toConfigProvider(p *domain.Provider) config.Provider {
+	return config.Provider{
+		ID:      p.ID,
+		Name:    p.Name,
+		APIKey:  p.APIKey,
+		BaseURL: p.BaseURL,
+		Enabled: p.Enabled,
+	}
+}