fix: new_visitors 排爬虫+日界/爬虫口径回归测试

- refresh_daily new_visitors 加 is_bot=False（爬虫不是受众，四指标口径统一）
- test_refresh_daily_shanghai_boundary：固定 UTC 时间戳钉死上海自然日界
- test_refresh_daily_excludes_bots：事件/会话/访客 is_bot 均不计入
- refresh_stats 删未使用的 stat = 赋值

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

server/tracker/management/commands/refresh_stats.py

@@ -19,6 +19,6 @@ class Command(BaseCommand):
         total = 0
         for site in Site.objects.filter(is_active=True):
             for i in range(opts["days"]):
-                stat = refresh_daily(site, today - timedelta(days=i))
+                refresh_daily(site, today - timedelta(days=i))
                 total += 1
         self.stdout.write(f"refreshed {total} site-days")