feat: 添加启动任务开关，修复统计聚合内存泄漏

- 新增 CACHE_WARMUP_ENABLED 和 MAINTENANCE_STARTUP_TASKS_ENABLED 环境变量，
  允许禁用缓存预热和维护调度器启动任务
- 在统计聚合批量处理循环中添加 db.expunge_all()，释放 Session identity map，
  防止 ORM 对象累积导致内存暴涨
- 添加启动任务开关的单元测试

tests/services/test_startup_toggles.py

@@ -0,0 +1,86 @@
+from __future__ import annotations
+
+import inspect
+from types import SimpleNamespace
+
+import pytest
+
+import src.services.system.cache_warmup as cache_warmup_module
+import src.services.system.maintenance_scheduler as maintenance_scheduler_module
+from src.config.settings import config
+from src.services.system.maintenance_scheduler import MaintenanceScheduler
+
+
+@pytest.mark.asyncio
+async def test_start_cache_warmup_skips_task_when_disabled(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(config, "cache_warmup_enabled", False)
+
+    created = False
+
+    def fake_create_task(coro):  # type: ignore[no-untyped-def]
+        nonlocal created
+        created = True
+        if inspect.iscoroutine(coro):
+            coro.close()
+        return object()
+
+    monkeypatch.setattr(cache_warmup_module.asyncio, "create_task", fake_create_task)
+
+    await cache_warmup_module.start_cache_warmup()
+
+    assert created is False
+
+
+@pytest.mark.asyncio
+async def test_start_cache_warmup_creates_task_when_enabled(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(config, "cache_warmup_enabled", True)
+
+    created = False
+
+    def fake_create_task(coro):  # type: ignore[no-untyped-def]
+        nonlocal created
+        created = True
+        if inspect.iscoroutine(coro):
+            coro.close()
+        return object()
+
+    monkeypatch.setattr(cache_warmup_module.asyncio, "create_task", fake_create_task)
+
+    await cache_warmup_module.start_cache_warmup()
+
+    assert created is True
+
+
+@pytest.mark.asyncio
+async def test_maintenance_scheduler_start_skips_startup_task_when_disabled(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(config, "maintenance_startup_tasks_enabled", False)
+
+    scheduler = MaintenanceScheduler()
+
+    created = False
+
+    def fake_create_task(coro):  # type: ignore[no-untyped-def]
+        nonlocal created
+        created = True
+        if inspect.iscoroutine(coro):
+            coro.close()
+        return object()
+
+    monkeypatch.setattr(maintenance_scheduler_module.asyncio, "create_task", fake_create_task)
+    monkeypatch.setattr(scheduler, "_get_checkin_time", lambda: (1, 5))
+    monkeypatch.setattr(
+        maintenance_scheduler_module,
+        "get_scheduler",
+        lambda: SimpleNamespace(
+            add_cron_job=lambda *args, **kwargs: None,
+            add_interval_job=lambda *args, **kwargs: None,
+        ),
+    )
+
+    await scheduler.start()
+
+    assert created is False