perf(assignments): 优化分配管理页面加载性能与角标数量展示

- 前端改造为服务端分页查询 (accountApi.listPaged)，避免一次性拉取 1.6 万+ 账号导致页面卡死
- 优化后端 assignments_summary 聚合查询，消除大子查询临时表
- 修复 Badge 默认 99+ 限制，设置 overflowCount 支持展示完整账号数量
- 修复测试用例 test_business_events.py 当前时间跨天断言问题

tests/test_business_events.py

@@ -454,8 +454,9 @@ class TestPointsExpireJob(_Base):
     def test_expired_points_are_recorded_once(self):
         from datetime import timedelta
 
+        now = datetime.now(UTC)
         self.account.points = 500
-        self.account.points_expire_at = NOW - timedelta(days=1)
+        self.account.points_expire_at = now - timedelta(days=1)
         self.session.commit()
 
         out = self._run()
@@ -472,8 +473,9 @@ class TestPointsExpireJob(_Base):
     def test_not_yet_expired_is_left_alone(self):
         from datetime import timedelta
 
+        now = datetime.now(UTC)
         self.account.points = 500
-        self.account.points_expire_at = NOW + timedelta(days=3)
+        self.account.points_expire_at = now + timedelta(days=3)
         self.session.commit()
 
         assert self._run()["written"] == 0
@@ -484,8 +486,9 @@ class TestPointsExpireJob(_Base):
         """积分已经为 0 就没有东西可到期 —— 写一条 -0 只是噪声。"""
         from datetime import timedelta
 
+        now = datetime.now(UTC)
         self.account.points = 0
-        self.account.points_expire_at = NOW - timedelta(days=1)
+        self.account.points_expire_at = now - timedelta(days=1)
         self.session.commit()
 
         out = self._run()