refactor: 全面集成 slog 日志到 handler、service、middleware

- cmd/main.go: 集成 logger.Init 和日志中间件，替换标准 log 包
- handler 层: 5xx 错误记录完整日志，返回通用消息（防内部信息泄露）
- service 层: LLM/图片生成/存储/认证等关键操作补充结构化日志
- auth 中间件: 记录认证失败原因
- generate.go: 后台管线任务使用带 task_id 的 logger

backend/internal/service/auth.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"gen2d/internal/db"
+	"gen2d/internal/logger"
 	"gen2d/internal/model"
 
 	"github.com/golang-jwt/jwt/v5"
@@ -29,8 +30,11 @@ func NewAuthService(jwtSecret string, jwtExpire int64) *AuthService {
 
 // Register 注册新用户，密码使用 bcrypt 加密存储。
 func (s *AuthService) Register(ctx context.Context, username, password, email string) (*model.User, error) {
+	l := logger.FromCtx(ctx)
+
 	hashed, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
 	if err != nil {
+		l.Error("bcrypt failed", "error", err)
 		return nil, err
 	}
 
@@ -42,25 +46,33 @@ func (s *AuthService) Register(ctx context.Context, username, password, email st
 
 	if err := db.DB.WithContext(ctx).Create(&user).Error; err != nil {
 		if errors.Is(err, gorm.ErrDuplicatedKey) {
+			l.Warn("register failed: duplicate username", "username", username)
 			return nil, errors.New("用户名已存在")
 		}
+		l.Error("register db failed", "username", username, "error", err)
 		return nil, err
 	}
 
+	l.Info("user registered", "username", username, "user_id", user.ID)
 	return &user, nil
 }
 
 // Login 校验用户名密码，成功返回 JWT token 及用户信息。
 func (s *AuthService) Login(ctx context.Context, username, password string) (string, int64, *model.User, error) {
+	l := logger.FromCtx(ctx)
+
 	var user model.User
 	if err := db.DB.WithContext(ctx).Where("username = ?", username).First(&user).Error; err != nil {
 		if errors.Is(err, gorm.ErrRecordNotFound) {
+			l.Warn("login failed: user not found", "username", username)
 			return "", 0, nil, errors.New("用户名或密码错误")
 		}
+		l.Error("login db query failed", "username", username, "error", err)
 		return "", 0, nil, err
 	}
 
 	if err := bcrypt.CompareHashAndPassword([]byte(user.Password), []byte(password)); err != nil {
+		l.Warn("login failed: wrong password", "username", username)
 		return "", 0, nil, errors.New("用户名或密码错误")
 	}
 
@@ -72,8 +84,10 @@ func (s *AuthService) Login(ctx context.Context, username, password string) (str
 	}
 	token, err := jwt.NewWithClaims(jwt.SigningMethodHS256, claims).SignedString(s.jwtSecret)
 	if err != nil {
+		l.Error("jwt sign failed", "error", err)
 		return "", 0, nil, err
 	}
 
+	l.Info("login succeeded", "username", username, "user_id", user.ID)
 	return token, int64(s.jwtExpire.Seconds()), &user, nil
 }