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/pkg/errors/errors.go

@@ -0,0 +1,74 @@
+package errors
+
+import (
+	"fmt"
+	"net/http"
+)
+
+// AppError 结构化应用错误
+type AppError struct {
+	Code       string                 `json:"code"`
+	Message    string                 `json:"message"`
+	HTTPStatus int                    `json:"-"`
+	Cause      error                  `json:"-"`
+	Context    map[string]interface{} `json:"-"`
+}
+
+// Error implements error interface
+func (e *AppError) Error() string {
+	if e.Cause != nil {
+		return fmt.Sprintf("%s: %s (%v)", e.Code, e.Message, e.Cause)
+	}
+	return fmt.Sprintf("%s: %s", e.Code, e.Message)
+}
+
+// Unwrap returns the underlying error
+func (e *AppError) Unwrap() error {
+	return e.Cause
+}
+
+// NewAppError creates a new AppError
+func NewAppError(code, message string, httpStatus int) *AppError {
+	return &AppError{
+		Code:       code,
+		Message:    message,
+		HTTPStatus: httpStatus,
+	}
+}
+
+// Predefined errors
+var (
+	ErrModelNotFound    = NewAppError("model_not_found", "模型未找到", http.StatusNotFound)
+	ErrModelDisabled    = NewAppError("model_disabled", "模型已禁用", http.StatusNotFound)
+	ErrProviderNotFound = NewAppError("provider_not_found", "供应商未找到", http.StatusNotFound)
+	ErrProviderDisabled = NewAppError("provider_disabled", "供应商已禁用", http.StatusNotFound)
+	ErrInvalidRequest   = NewAppError("invalid_request", "无效的请求", http.StatusBadRequest)
+	ErrInternal         = NewAppError("internal_error", "内部错误", http.StatusInternalServerError)
+	ErrDatabaseNotInit  = NewAppError("database_not_initialized", "数据库未初始化", http.StatusInternalServerError)
+	ErrConflict         = NewAppError("conflict", "资源已存在", http.StatusConflict)
+)
+
+// AsAppError 尝试将 error 转换为 *AppError
+func AsAppError(err error) (*AppError, bool) {
+	if err == nil {
+		return nil, false
+	}
+	var appErr *AppError
+	if ok := is(err, &appErr); ok {
+		return appErr, true
+	}
+	return nil, false
+}
+
+func is(err error, target interface{}) bool {
+	// 简单的类型断言
+	if e, ok := err.(*AppError); ok {
+		// 直接赋值
+		switch t := target.(type) {
+		case **AppError:
+			*t = e
+			return true
+		}
+	}
+	return false
+}