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/mildware/auth.go

@@ -4,6 +4,7 @@ import (
 	"net/http"
 	"strings"
 
+	"gen2d/internal/logger"
 	"gen2d/internal/model"
 
 	"github.com/gin-gonic/gin"
@@ -15,12 +16,14 @@ func AuthMiddleware(jwtSecret string) gin.HandlerFunc {
 	return func(c *gin.Context) {
 		authHeader := c.GetHeader("Authorization")
 		if authHeader == "" {
+			logger.FromCtx(c.Request.Context()).Warn("auth failed: no token")
 			c.AbortWithStatusJSON(http.StatusUnauthorized, model.Fail(http.StatusUnauthorized, "未提供认证令牌"))
 			return
 		}
 
 		tokenString := strings.TrimPrefix(authHeader, "Bearer ")
 		if tokenString == authHeader {
+			logger.FromCtx(c.Request.Context()).Warn("auth failed: invalid format")
 			c.AbortWithStatusJSON(http.StatusUnauthorized, model.Fail(http.StatusUnauthorized, "认证格式错误，需为 Bearer <token>"))
 			return
 		}
@@ -29,12 +32,14 @@ func AuthMiddleware(jwtSecret string) gin.HandlerFunc {
 			return []byte(jwtSecret), nil
 		})
 		if err != nil || !token.Valid {
+			logger.FromCtx(c.Request.Context()).Warn("auth failed: invalid token", "error", err)
 			c.AbortWithStatusJSON(http.StatusUnauthorized, model.Fail(http.StatusUnauthorized, "令牌无效或已过期"))
 			return
 		}
 
 		claims, ok := token.Claims.(jwt.MapClaims)
 		if !ok {
+			logger.FromCtx(c.Request.Context()).Warn("auth failed: parse claims")
 			c.AbortWithStatusJSON(http.StatusUnauthorized, model.Fail(http.StatusUnauthorized, "令牌解析失败"))
 			return
 		}