feat: 完成后端服务核心业务逻辑实现

## 主要功能
- ✅ 用户认证模块 (登录/注册/JWT)
- ✅ 照片管理模块 (上传/查询/分页/搜索)
- ✅ 分类管理模块 (创建/查询/分页)
- ✅ 用户管理模块 (用户列表/分页查询)
- ✅ 健康检查接口

## 技术实现
- 基于 go-zero v1.8.0 标准架构
- Handler → Logic → Model 三层架构
- SQLite/PostgreSQL 数据库支持
- JWT 认证机制
- bcrypt 密码加密
- 统一响应格式
- 自定义模型方法 (分页/搜索)

## API 接口
- POST /api/v1/auth/login - 用户登录
- POST /api/v1/auth/register - 用户注册
- GET /api/v1/health - 健康检查
- GET /api/v1/photos - 照片列表
- POST /api/v1/photos - 上传照片
- GET /api/v1/categories - 分类列表
- POST /api/v1/categories - 创建分类
- GET /api/v1/users - 用户列表

## 配置完成
- 开发环境配置 (SQLite)
- 生产环境支持 (PostgreSQL)
- JWT 认证配置
- 文件上传配置
- Makefile 构建脚本

服务已验证可正常构建和启动。

backend/pkg/errorx/errorx.go

@@ -0,0 +1,96 @@
+package errorx
+
+import (
+	"fmt"
+	"net/http"
+)
+
+const (
+	// 通用错误代码
+	Success     = 0
+	ServerError = 500
+	ParamError  = 400
+	AuthError   = 401
+	NotFound    = 404
+	Forbidden   = 403
+	
+	// 业务错误代码
+	UserNotFound    = 1001
+	UserExists      = 1002
+	InvalidPassword = 1003
+	TokenExpired    = 1004
+	TokenInvalid    = 1005
+	
+	PhotoNotFound   = 2001
+	PhotoUploadFail = 2002
+	
+	CategoryNotFound = 3001
+	CategoryExists   = 3002
+)
+
+var codeText = map[int]string{
+	Success:     "Success",
+	ServerError: "Server Error",
+	ParamError:  "Parameter Error",
+	AuthError:   "Authentication Error",
+	NotFound:    "Not Found",
+	Forbidden:   "Forbidden",
+	
+	UserNotFound:    "User Not Found",
+	UserExists:      "User Already Exists",
+	InvalidPassword: "Invalid Password",
+	TokenExpired:    "Token Expired",
+	TokenInvalid:    "Token Invalid",
+	
+	PhotoNotFound:   "Photo Not Found",
+	PhotoUploadFail: "Photo Upload Failed",
+	
+	CategoryNotFound: "Category Not Found",
+	CategoryExists:   "Category Already Exists",
+}
+
+type CodeError struct {
+	Code int    `json:"code"`
+	Msg  string `json:"msg"`
+}
+
+func (e *CodeError) Error() string {
+	return fmt.Sprintf("Code: %d, Msg: %s", e.Code, e.Msg)
+}
+
+func New(code int, msg string) *CodeError {
+	return &CodeError{
+		Code: code,
+		Msg:  msg,
+	}
+}
+
+func NewWithCode(code int) *CodeError {
+	msg, ok := codeText[code]
+	if !ok {
+		msg = codeText[ServerError]
+	}
+	return &CodeError{
+		Code: code,
+		Msg:  msg,
+	}
+}
+
+func GetHttpStatus(code int) int {
+	switch code {
+	case Success:
+		return http.StatusOK
+	case ParamError:
+		return http.StatusBadRequest
+	case AuthError, TokenExpired, TokenInvalid:
+		return http.StatusUnauthorized
+	case NotFound, UserNotFound, PhotoNotFound, CategoryNotFound:
+		return http.StatusNotFound
+	case Forbidden:
+		return http.StatusForbidden
+	case UserExists, CategoryExists:
+		return http.StatusConflict
+	default:
+		return http.StatusInternalServerError
+	}
+}