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_test.go

@@ -0,0 +1,125 @@
+package errors
+
+import (
+	"errors"
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewAppError(t *testing.T) {
+	err := NewAppError("test_code", "测试消息", http.StatusBadRequest)
+	assert.Equal(t, "test_code", err.Code)
+	assert.Equal(t, "测试消息", err.Message)
+	assert.Equal(t, http.StatusBadRequest, err.HTTPStatus)
+	assert.Nil(t, err.Cause)
+	assert.Nil(t, err.Context)
+}
+
+func TestAppError_Error(t *testing.T) {
+	tests := []struct {
+		name     string
+		err      *AppError
+		expected string
+	}{
+		{
+			name:     "无原因错误",
+			err:      NewAppError("code1", "消息1", 400),
+			expected: "code1: 消息1",
+		},
+		{
+			name:     "带原因错误",
+			err:      Wrap(NewAppError("code2", "消息2", 500), errors.New("原始错误")),
+			expected: "code2: 消息2 (原始错误)",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.expected, tt.err.Error())
+		})
+	}
+}
+
+func TestAppError_Unwrap(t *testing.T) {
+	cause := errors.New("原始错误")
+	err := Wrap(ErrInternal, cause)
+	assert.Equal(t, cause, err.Unwrap())
+}
+
+func TestWrap(t *testing.T) {
+	cause := errors.New("网络超时")
+	wrapped := Wrap(ErrInternal, cause)
+	assert.Equal(t, "internal_error", wrapped.Code)
+	assert.Equal(t, "内部错误", wrapped.Message)
+	assert.Equal(t, http.StatusInternalServerError, wrapped.HTTPStatus)
+	assert.Equal(t, cause, wrapped.Cause)
+}
+
+func TestWithContext(t *testing.T) {
+	err := WithContext(ErrModelNotFound, "model", "gpt-4")
+	assert.Equal(t, "model_not_found", err.Code)
+	assert.NotNil(t, err.Context)
+	assert.Equal(t, "gpt-4", err.Context["model"])
+
+	// 测试链式添加上下文
+	err2 := WithContext(err, "provider", "openai")
+	assert.Equal(t, "gpt-4", err2.Context["model"])
+	assert.Equal(t, "openai", err2.Context["provider"])
+}
+
+func TestWithMessage(t *testing.T) {
+	err := WithMessage(ErrInvalidRequest, "自定义错误消息")
+	assert.Equal(t, "invalid_request", err.Code)
+	assert.Equal(t, "自定义错误消息", err.Message)
+	assert.Equal(t, http.StatusBadRequest, err.HTTPStatus)
+}
+
+func TestPredefinedErrors(t *testing.T) {
+	tests := []struct {
+		name       string
+		err        *AppError
+		code       string
+		httpStatus int
+	}{
+		{"ErrModelNotFound", ErrModelNotFound, "model_not_found", http.StatusNotFound},
+		{"ErrModelDisabled", ErrModelDisabled, "model_disabled", http.StatusNotFound},
+		{"ErrProviderNotFound", ErrProviderNotFound, "provider_not_found", http.StatusNotFound},
+		{"ErrProviderDisabled", ErrProviderDisabled, "provider_disabled", http.StatusNotFound},
+		{"ErrInvalidRequest", ErrInvalidRequest, "invalid_request", http.StatusBadRequest},
+		{"ErrInternal", ErrInternal, "internal_error", http.StatusInternalServerError},
+		{"ErrDatabaseNotInit", ErrDatabaseNotInit, "database_not_initialized", http.StatusInternalServerError},
+		{"ErrConflict", ErrConflict, "conflict", http.StatusConflict},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.code, tt.err.Code)
+			assert.Equal(t, tt.httpStatus, tt.err.HTTPStatus)
+		})
+	}
+}
+
+func TestAsAppError(t *testing.T) {
+	t.Run("nil输入", func(t *testing.T) {
+		_, ok := AsAppError(nil)
+		assert.False(t, ok)
+	})
+
+	t.Run("AppError类型", func(t *testing.T) {
+		appErr, ok := AsAppError(ErrModelNotFound)
+		assert.True(t, ok)
+		assert.Equal(t, ErrModelNotFound, appErr)
+	})
+
+	t.Run("Wrapped AppError", func(t *testing.T) {
+		wrapped := Wrap(ErrInternal, errors.New("cause"))
+		appErr, ok := AsAppError(wrapped)
+		assert.True(t, ok)
+		assert.Equal(t, "internal_error", appErr.Code)
+	})
+
+	t.Run("非AppError类型", func(t *testing.T) {
+		_, ok := AsAppError(errors.New("普通错误"))
+		assert.False(t, ok)
+	})
+}