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/prompt_agent.go

@@ -7,11 +7,11 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
-	"log"
 	"net/http"
 	"strings"
 
 	"gen2d/internal/config"
+	"gen2d/internal/logger"
 
 	"github.com/cloudwego/eino/compose"
 )
@@ -130,8 +130,10 @@ type chatResponse struct {
 
 // callLLMRefine 调用 LLM 生成规范化提示词。未配置 API key 时回退到模板生成。
 func callLLMRefine(ctx context.Context, metaPrompt string) (PromptAgentOutput, error) {
+	l := logger.FromCtx(ctx)
+
 	if llmCfg.APIKey == "" {
-		log.Println("[prompt_agent] LLM API key not configured, using template fallback")
+		l.Warn("LLM API key not configured, using template fallback")
 		return fallbackRefine(metaPrompt), nil
 	}
 
@@ -140,12 +142,14 @@ func callLLMRefine(ctx context.Context, metaPrompt string) (PromptAgentOutput, e
 		{Role: "user", Content: metaPrompt},
 	}
 
+	l.Info("calling LLM API", "model", llmCfg.Model)
 	result, err := chatCompletion(ctx, messages)
 	if err != nil {
-		log.Printf("[prompt_agent] LLM API call failed: %v, using template fallback", err)
+		l.Error("LLM API call failed, using template fallback", "error", err)
 		return fallbackRefine(metaPrompt), nil
 	}
 
+	l.Info("LLM API succeeded", "response_length", len(result))
 	return PromptAgentOutput{
 		Prompt:  result,
 		RawText: result,
@@ -175,6 +179,8 @@ func chatCompletion(ctx context.Context, messages []chatMessage) (string, error)
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Set("Authorization", "Bearer "+llmCfg.APIKey)
 
+	l := logger.FromCtx(ctx)
+
 	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
 		return "", fmt.Errorf("send request: %w", err)
@@ -183,6 +189,7 @@ func chatCompletion(ctx context.Context, messages []chatMessage) (string, error)
 
 	if resp.StatusCode != http.StatusOK {
 		b, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
+		l.Error("LLM API error", "status", resp.StatusCode, "body", string(b))
 		return "", fmt.Errorf("llm api error %d: %s", resp.StatusCode, string(b))
 	}