feat: 日聚合服务与命令 + 索引迁移打包（Task3 审查结论）

- refresh_daily：按上海自然日聚合 pv/uv/sessions/new_visitors，update_or_create 幂等
- refresh_stats 管理命令：回刷近 N 天（生产由 1Panel 每日 00:10 调度，见 Task 22）
- 索引打包（0002）：Event/VisitSession 增加 (site, 时间) 日聚合索引；
  删 5 个冗余单列 FK 索引（Event.site/session、VisitSession.site/visitor、DailyStat.site），
  保留 Event.visitor/Visitor.site；sqlite PRAGMA 实证级联索引完整、EXPLAIN 走新索引
- test_models：IntegrityError 断言收紧 + Site 级联删除测试

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

server/tracker/tests/test_models.py

@@ -1,4 +1,5 @@
 import pytest
+from django.db import IntegrityError
 from tracker.models import Site, Visitor, VisitSession, Event
 
 pytestmark = pytest.mark.django_db
@@ -22,6 +23,16 @@ def test_event_unique_site_event_id():
         started_at="2026-09-15T10:00:00Z", ended_at="2026-09-15T10:01:00Z")
     Event.objects.create(site=site, visitor=v, session=s, event_id="e1",
         event_name="page_view", occurred_at="2026-09-15T10:00:00Z", path="/")
-    with pytest.raises(Exception):
+    with pytest.raises(IntegrityError):
         Event.objects.create(site=site, visitor=v, session=s, event_id="e1",
             event_name="page_view", occurred_at="2026-09-15T10:01:00Z", path="/")
+
+def test_site_cascade_deletes_children():
+    site = Site.objects.create(name="s", domains=["a.com"])
+    v = Visitor.objects.create(site=site, visitor_id="v")
+    s = VisitSession.objects.create(site=site, visitor=v, session_id="s",
+        started_at="2026-09-15T10:00:00Z", ended_at="2026-09-15T10:01:00Z")
+    Event.objects.create(site=site, visitor=v, session=s, event_id="e",
+        event_name="page_view", occurred_at="2026-09-15T10:00:00Z", path="/")
+    site.delete()
+    assert Visitor.objects.count() == 0 and VisitSession.objects.count() == 0 and Event.objects.count() == 0