perf(日志): 降噪优化，将常规成功日志降级为 Debug 级别

- GIN Logger 中间件跳过 /health 和 /setup/status 的请求日志
- UsageCleanup 空闲轮询（no_task）日志降级为 slog.Debug
- Scheduler 常规 rebuild ok 日志降级为 slog.Debug
- DashboardAggregation 常规聚合完成日志降级为 slog.Debug
- TokenRefresh 无刷新活动时周期日志降级为 slog.Debug

生产环境（Info 级别）下自动静默，debug 模式下仍可见。
错误、警告类日志保持原有级别不变。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/scheduler_snapshot_service.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"log"
+	"log/slog"
 	"strconv"
 	"sync"
 	"time"
@@ -451,7 +452,7 @@ func (s *SchedulerSnapshotService) rebuildBucket(ctx context.Context, bucket Sch
 		log.Printf("[Scheduler] rebuild cache failed: bucket=%s reason=%s err=%v", bucket.String(), reason, err)
 		return err
 	}
-	log.Printf("[Scheduler] rebuild ok: bucket=%s reason=%s size=%d", bucket.String(), reason, len(accounts))
+	slog.Debug("[Scheduler] rebuild ok", "bucket", bucket.String(), "reason", reason, "size", len(accounts))
 	return nil
 }