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/handler/generate.go

@@ -3,11 +3,11 @@ package handler
 import (
 	"context"
 	"fmt"
-	"log"
 	"net/http"
 	"sync"
 	"time"
 
+	"gen2d/internal/logger"
 	"gen2d/internal/model"
 	"gen2d/internal/service"
 
@@ -55,8 +55,8 @@ type TaskResponse struct {
 
 // AssetsResponse 素材列表响应。
 type AssetsResponse struct {
-	Assets   []AssetResponse        `json:"assets"`
-	Metadata service.AssetMetadata  `json:"metadata"`
+	Assets   []AssetResponse       `json:"assets"`
+	Metadata service.AssetMetadata `json:"metadata"`
 }
 
 // taskRecord 内存中的任务记录。
@@ -103,11 +103,13 @@ func Generate(c *gin.Context) {
 	c.JSON(http.StatusOK, model.OK(GenerateResponse{TaskID: taskID}))
 
 	// 后台执行管线
-	go runPipelineBg(projectID, taskID, req)
+	go runPipelineBg(c.Request.Context(), projectID, taskID, req)
 }
 
 // runPipelineBg 后台执行生成管线，更新任务状态。
-func runPipelineBg(projectID, taskID string, req GenerateRequest) {
+func runPipelineBg(ctx context.Context, projectID, taskID string, req GenerateRequest) {
+	l := logger.With("task_id", taskID, "project_id", projectID)
+
 	updateStatus(taskID, "running", 10)
 
 	in := service.PipelineInput{
@@ -129,11 +131,10 @@ func runPipelineBg(projectID, taskID string, req GenerateRequest) {
 		},
 	}
 
-	ctx := context.Background()
 	output, err := service.RunPipeline(ctx, in)
 	if err != nil {
-		log.Printf("[generate] task %s failed: %v", taskID, err)
-		updateFailed(taskID, err.Error())
+		l.Error("task pipeline failed", "error", err)
+		updateFailed(taskID, "生成管线执行失败")
 		return
 	}
 
@@ -144,7 +145,8 @@ func runPipelineBg(projectID, taskID string, req GenerateRequest) {
 		key := fmt.Sprintf("generation/%s/%s/%d.%s", projectID, taskID, i, a.Format)
 		cdnURL, err := storageSvc.Upload(ctx, key, a.Data)
 		if err != nil {
-			updateFailed(taskID, "上传素材失败: "+err.Error())
+			l.Error("upload asset failed", "index", i, "error", err)
+			updateFailed(taskID, "上传素材失败")
 			return
 		}
 		assets[i] = AssetResponse{
@@ -168,7 +170,7 @@ func runPipelineBg(projectID, taskID string, req GenerateRequest) {
 		metadata: output.Metadata,
 	})
 
-	log.Printf("[generate] task %s completed, %d assets", taskID, len(assets))
+	l.Info("task completed", "asset_count", len(assets))
 }
 
 func updateStatus(taskID, status string, progress int) {