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/inference.go

@@ -11,13 +11,13 @@ import (
 	"image/color"
 	"image/png"
 	"io"
-	"log"
 	"mime/multipart"
 	"net/http"
 	"strings"
 	"time"
 
 	"gen2d/internal/config"
+	"gen2d/internal/logger"
 )
 
 // imgCfg 保存文生图配置，由 main 通过 InitImageGenConfig 注入。
@@ -64,21 +64,33 @@ func GenerateImages(ctx context.Context, prompt string, params AssetParams) ([]G
 		count = params.Frames.Directions * params.Frames.FramesPerDirection
 	}
 
+	l := logger.FromCtx(ctx)
+
 	if imgCfg.APIKey != "" {
 		width, height := imgCfg.Width, imgCfg.Height
 		if params.Resolution > 0 {
 			width = params.Resolution
 			height = params.Resolution
 		}
-		log.Println("[inference] calling image gen API")
-		return callImageAPI(ctx, prompt, count, width, height)
+		l.Info("calling image gen API",
+			"model", imgCfg.Model,
+			"count", count,
+			"size", fmt.Sprintf("%dx%d", width, height),
+		)
+		images, err := callImageAPI(ctx, prompt, count, width, height)
+		if err != nil {
+			l.Error("image gen API failed", "error", err)
+			return nil, err
+		}
+		l.Info("image gen API succeeded", "image_count", len(images))
+		return images, nil
 	}
 
 	size := params.Resolution
 	if size <= 0 {
 		size = 64
 	}
-	log.Println("[inference] image API key not configured, using mock")
+	l.Warn("image API key not configured, using mock")
 	return generateMockImages(size, count)
 }
 
@@ -107,10 +119,15 @@ func callImageAPI(ctx context.Context, prompt string, count, width, height int)
 		retryDelay = 5 * time.Second
 	}
 
+	l := logger.FromCtx(ctx)
+
 	var lastErr error
 	for attempt := 0; attempt <= maxRetries; attempt++ {
 		if attempt > 0 {
-			log.Printf("[inference] retrying image API (attempt %d/%d)", attempt, maxRetries)
+			l.Warn("retrying image API",
+				"attempt", attempt,
+				"max_retries", maxRetries,
+			)
 			select {
 			case <-ctx.Done():
 				return nil, fmt.Errorf("context cancelled during retry: %w", ctx.Err())
@@ -128,6 +145,7 @@ func callImageAPI(ctx context.Context, prompt string, count, width, height int)
 		resp, err := imageHTTPClient.Do(req)
 		if err != nil {
 			lastErr = fmt.Errorf("send request: %w", err)
+			l.Warn("image API request failed", "error", lastErr)
 			continue
 		}
 
@@ -140,8 +158,18 @@ func callImageAPI(ctx context.Context, prompt string, count, width, height int)
 
 		if resp.StatusCode >= 500 {
 			lastErr = fmt.Errorf("image api error %d: %s", resp.StatusCode, string(b))
+			l.Error("image API server error",
+				"status", resp.StatusCode,
+				"body", string(b),
+				"attempt", attempt,
+			)
 			continue
 		}
+		// 4xx 不重试
+		l.Error("image API client error",
+			"status", resp.StatusCode,
+			"body", string(b),
+		)
 		return nil, fmt.Errorf("image api error %d: %s", resp.StatusCode, string(b))
 	}
 
@@ -205,10 +233,17 @@ func downloadImage(ctx context.Context, url string) ([]byte, error) {
 
 // EditImages 图片编辑接口，调用 OpenAI 兼容 Images Edits API（multipart/form-data）。
 func EditImages(ctx context.Context, imageData []byte, prompt string, count int) ([]GeneratedImage, error) {
+	l := logger.FromCtx(ctx)
+
 	if imgCfg.APIKey == "" {
 		return nil, fmt.Errorf("image API key not configured")
 	}
 
+	l.Info("calling image edit API",
+		"model", imgCfg.Model,
+		"count", count,
+	)
+
 	var buf bytes.Buffer
 	writer := multipart.NewWriter(&buf)
 
@@ -239,16 +274,25 @@ func EditImages(ctx context.Context, imageData []byte, prompt string, count int)
 
 	resp, err := imageHTTPClient.Do(req)
 	if err != nil {
+		l.Error("image edit API request failed", "error", err)
 		return nil, fmt.Errorf("send request: %w", err)
 	}
 	defer resp.Body.Close()
 
 	if resp.StatusCode != http.StatusOK {
 		b, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
+		l.Error("image edit API error", "status", resp.StatusCode, "body", string(b))
 		return nil, fmt.Errorf("image edit api error %d: %s", resp.StatusCode, string(b))
 	}
 
-	return parseImageResponse(ctx, resp.Body, imgCfg.Width, imgCfg.Height)
+	images, err := parseImageResponse(ctx, resp.Body, imgCfg.Width, imgCfg.Height)
+	if err != nil {
+		l.Error("image edit response parse failed", "error", err)
+		return nil, err
+	}
+
+	l.Info("image edit API succeeded", "image_count", len(images))
+	return images, nil
 }
 
 // ======================== 质检 ========================