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/token_refresh_service.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"log/slog"
 	"strings"
 	"sync"
 	"time"
@@ -164,9 +165,15 @@ func (s *TokenRefreshService) processRefresh() {
 		}
 	}
 
-	// 始终打印周期日志，便于跟踪服务运行状态
-	log.Printf("[TokenRefresh] Cycle complete: total=%d, oauth=%d, needs_refresh=%d, refreshed=%d, failed=%d",
-		totalAccounts, oauthAccounts, needsRefresh, refreshed, failed)
+	// 无刷新活动时降级为 Debug，有实际刷新活动时保持 Info
+	if needsRefresh == 0 && failed == 0 {
+		slog.Debug("[TokenRefresh] Cycle complete",
+			"total", totalAccounts, "oauth", oauthAccounts,
+			"needs_refresh", needsRefresh, "refreshed", refreshed, "failed", failed)
+	} else {
+		log.Printf("[TokenRefresh] Cycle complete: total=%d, oauth=%d, needs_refresh=%d, refreshed=%d, failed=%d",
+			totalAccounts, oauthAccounts, needsRefresh, refreshed, failed)
+	}
 }
 
 // listActiveAccounts 获取所有active状态的账号