refactor: 后端代码质量优化 - 复用公共库、使用标准库、类型安全错误判断

## 高优先级修复
- stats_service_impl: 使用 strings.SplitN 替代错误的索引分割
- provider_handler: 使用 errors.Is(err, gorm.ErrDuplicatedKey) 替代字符串匹配
- client: 重写 isNetworkError 使用 errors.As/Is 类型安全判断
- proxy_handler: 使用 encoding/json 标准库解析 JSON（extractModelName、isStreamRequest）

## 中优先级修复
- stats_handler: 添加 parseDateParam 辅助函数消除重复日期解析
- pkg/errors: 新增 ErrRequestCreate/Send/ResponseRead 错误类型和 WithCause 方法
- client: 使用结构化错误替代 fmt.Errorf
- ConversionEngine: logger 依赖注入，替换所有 zap.L() 调用

## 低优先级修复
- encoder: 删除 joinStrings，使用 strings.Join
- adapter: 删除 modelInfoRegex 正则，使用 isModelInfoPath 字符串函数

## 文档更新
- README.md: 添加公共库使用指南和编码规范章节
- specs: 同步 delta specs 到 main specs（error-handling、structured-logging、request-validation）

## 归档
- openspec/changes/archive/2026-04-20-refactor-backend-code-quality/

backend/pkg/errors/errors.go

@@ -27,6 +27,17 @@ func (e *AppError) Unwrap() error {
 	return e.Cause
 }
 
+// WithCause returns a copy of the error with the given cause
+func (e *AppError) WithCause(cause error) *AppError {
+	return &AppError{
+		Code:       e.Code,
+		Message:    e.Message,
+		HTTPStatus: e.HTTPStatus,
+		Cause:      cause,
+		Context:    e.Context,
+	}
+}
+
 // NewAppError creates a new AppError
 func NewAppError(code, message string, httpStatus int) *AppError {
 	return &AppError{
@@ -46,6 +57,9 @@ var (
 	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)
+	ErrRequestCreate    = NewAppError("request_create_error", "创建请求失败", http.StatusInternalServerError)
+	ErrRequestSend      = NewAppError("request_send_error", "发送请求失败", http.StatusBadGateway)
+	ErrResponseRead     = NewAppError("response_read_error", "读取响应失败", http.StatusBadGateway)
 )
 
 // AsAppError 尝试将 error 转换为 *AppError

backend/pkg/errors/errors_test.go

@@ -90,6 +90,9 @@ func TestPredefinedErrors(t *testing.T) {
 		{"ErrInternal", ErrInternal, "internal_error", http.StatusInternalServerError},
 		{"ErrDatabaseNotInit", ErrDatabaseNotInit, "database_not_initialized", http.StatusInternalServerError},
 		{"ErrConflict", ErrConflict, "conflict", http.StatusConflict},
+		{"ErrRequestCreate", ErrRequestCreate, "request_create_error", http.StatusInternalServerError},
+		{"ErrRequestSend", ErrRequestSend, "request_send_error", http.StatusBadGateway},
+		{"ErrResponseRead", ErrResponseRead, "response_read_error", http.StatusBadGateway},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -123,3 +126,16 @@ func TestAsAppError(t *testing.T) {
 		assert.False(t, ok)
 	})
 }
+
+func TestWithCause(t *testing.T) {
+	cause := errors.New("连接超时")
+	err := ErrRequestSend.WithCause(cause)
+	assert.Equal(t, "request_send_error", err.Code)
+	assert.Equal(t, http.StatusBadGateway, err.HTTPStatus)
+	assert.Equal(t, cause, err.Cause)
+	assert.True(t, errors.Is(err, cause))
+
+	var appErr *AppError
+	assert.True(t, errors.As(err, &appErr))
+	assert.Equal(t, "request_send_error", appErr.Code)
+}