chore(logging): 完成后端日志审计与结构化迁移

- 将高密度服务与处理器日志迁移到新日志系统（LegacyPrintf/结构化日志）
- 增加 stdlog bridge 与兼容测试，保留旧日志捕获能力
- 将 OpenAI 断流告警改为结构化 Warn 并改造对应测试为 sink 捕获
- 补齐后端相关文件 logger 引用并通过全量 go test

backend/internal/service/dashboard_service.go

@@ -5,11 +5,11 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"log"
 	"sync/atomic"
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/logger"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/usagestats"
 )
 
@@ -113,7 +113,7 @@ func (s *DashboardService) GetDashboardStats(ctx context.Context) (*usagestats.D
 			return cached, nil
 		}
 		if err != nil && !errors.Is(err, ErrDashboardStatsCacheMiss) {
-			log.Printf("[Dashboard] 仪表盘缓存读取失败: %v", err)
+			logger.LegacyPrintf("service.dashboard", "[Dashboard] 仪表盘缓存读取失败: %v", err)
 		}
 	}
 
@@ -188,7 +188,7 @@ func (s *DashboardService) refreshDashboardStatsAsync() {
 
 		stats, err := s.fetchDashboardStats(ctx)
 		if err != nil {
-			log.Printf("[Dashboard] 仪表盘缓存异步刷新失败: %v", err)
+			logger.LegacyPrintf("service.dashboard", "[Dashboard] 仪表盘缓存异步刷新失败: %v", err)
 			return
 		}
 		s.applyAggregationStatus(ctx, stats)
@@ -220,12 +220,12 @@ func (s *DashboardService) saveDashboardStatsCache(ctx context.Context, stats *u
 	}
 	data, err := json.Marshal(entry)
 	if err != nil {
-		log.Printf("[Dashboard] 仪表盘缓存序列化失败: %v", err)
+		logger.LegacyPrintf("service.dashboard", "[Dashboard] 仪表盘缓存序列化失败: %v", err)
 		return
 	}
 
 	if err := s.cache.SetDashboardStats(ctx, string(data), s.cacheTTL); err != nil {
-		log.Printf("[Dashboard] 仪表盘缓存写入失败: %v", err)
+		logger.LegacyPrintf("service.dashboard", "[Dashboard] 仪表盘缓存写入失败: %v", err)
 	}
 }
 
@@ -237,10 +237,10 @@ func (s *DashboardService) evictDashboardStatsCache(reason error) {
 	defer cancel()
 
 	if err := s.cache.DeleteDashboardStats(cacheCtx); err != nil {
-		log.Printf("[Dashboard] 仪表盘缓存清理失败: %v", err)
+		logger.LegacyPrintf("service.dashboard", "[Dashboard] 仪表盘缓存清理失败: %v", err)
 	}
 	if reason != nil {
-		log.Printf("[Dashboard] 仪表盘缓存异常，已清理: %v", reason)
+		logger.LegacyPrintf("service.dashboard", "[Dashboard] 仪表盘缓存异常，已清理: %v", reason)
 	}
 }
 
@@ -271,7 +271,7 @@ func (s *DashboardService) fetchAggregationUpdatedAt(ctx context.Context) time.T
 	}
 	updatedAt, err := s.aggRepo.GetAggregationWatermark(ctx)
 	if err != nil {
-		log.Printf("[Dashboard] 读取聚合水位失败: %v", err)
+		logger.LegacyPrintf("service.dashboard", "[Dashboard] 读取聚合水位失败: %v", err)
 		return time.Unix(0, 0).UTC()
 	}
 	if updatedAt.IsZero() {