chore: dashboard 确定性排序与注释修正

- sources/top_pages Top8 加平局次序键（referrer_domain/path），
  平局时跨请求顺序稳定
- test_dashboard 修正过时注释（b 站数据同为 refresh_daily 真实落行）

pytest 79 passed 无回归。

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

server/tracker/api/dashboard.py

@@ -72,7 +72,8 @@ class OverviewView(EnvelopeMixin, APIView):
             q = q.filter(site=site)
         return [{"source": r["referrer_domain"] or "直接访问", "sessions": r["n"]}
                 for r in q.values("referrer_domain")
-                         .annotate(n=Count("id")).order_by("-n")[:8]]
+                         .annotate(n=Count("id"))
+                         .order_by("-n", "referrer_domain")[:8]]   # 平局时次序跨请求稳定
 
     def _today_pages(self, site, today):
         start, end = _day_bounds(today)
@@ -82,4 +83,5 @@ class OverviewView(EnvelopeMixin, APIView):
             q = q.filter(site=site)
         return [{"path": r["path"], "pv": r["n"]}
                 for r in q.values("path")
-                         .annotate(n=Count("id")).order_by("-n")[:8]]
+                         .annotate(n=Count("id"))
+                         .order_by("-n", "path")[:8]]   # 平局时次序跨请求稳定