feat(cleanup): 解耦 request_candidates 与 provider_api_keys 生命周期

- 移除 request_candidates.key_id 对 provider_api_keys 的外键约束(含迁移脚本)
- 删除 Key 时不再级联删除候选记录，改为独立按保留天数定时清理
- 新增 request_candidates_retention_days / request_candidates_cleanup_batch_size 配置项
- batch_delete_task 增加 lock_timeout 及超时自动降批重试机制
- cleanup_key_references 提取阶段化清理流程，移除 RequestCandidate 联动删除
- 前端 CleanupPolicySection 新增候选记录保留天数和清理批次配置

Closes #227

Co-authored-by: Entropy-Xu <entropy.xu@cloudhabitatsh.com>

tests/services/test_startup_toggles.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 import inspect
 from types import SimpleNamespace
+from unittest.mock import MagicMock
 
 import pytest
 
@@ -41,3 +42,76 @@ async def test_maintenance_scheduler_start_skips_startup_task_when_disabled(
     await scheduler.start()
 
     assert created is False
+
+
+@pytest.mark.asyncio
+async def test_candidate_cleanup_uses_dedicated_retention_and_batch_settings(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    scheduler = MaintenanceScheduler()
+
+    class _FakeLoop:
+        async def run_in_executor(self, _executor, func):  # type: ignore[no-untyped-def]
+            return func()
+
+    class _ConfigSession:
+        def close(self) -> None:
+            return None
+
+    class _BatchSession:
+        def __init__(self, ids: list[str]) -> None:
+            self.ids = ids
+            self.closed = False
+            self.committed = False
+            self.query_obj = MagicMock()
+            filtered = self.query_obj.filter.return_value
+            filtered.order_by.return_value.limit.return_value.all.return_value = [
+                SimpleNamespace(id=value) for value in ids
+            ]
+
+        def query(self, _model):  # type: ignore[no-untyped-def]
+            return self.query_obj
+
+        def execute(self, _statement):  # type: ignore[no-untyped-def]
+            return SimpleNamespace(rowcount=len(self.ids))
+
+        def commit(self) -> None:
+            self.committed = True
+
+        def rollback(self) -> None:
+            raise AssertionError("rollback should not be called")
+
+        def close(self) -> None:
+            self.closed = True
+
+    config_session = _ConfigSession()
+    batch_one = _BatchSession(["candidate-1", "candidate-2"])
+    batch_two = _BatchSession([])
+    sessions = iter([config_session, batch_one, batch_two])
+
+    def fake_create_session():  # type: ignore[no-untyped-def]
+        return next(sessions)
+
+    config_values = {
+        "enable_auto_cleanup": True,
+        "request_candidates_retention_days": 21,
+        "request_candidates_cleanup_batch_size": 2,
+    }
+
+    monkeypatch.setattr(maintenance_scheduler_module, "create_session", fake_create_session)
+    monkeypatch.setattr(
+        maintenance_scheduler_module.SystemConfigService,
+        "get_config",
+        lambda _db, key, default=None: config_values.get(key, default),
+    )
+    monkeypatch.setattr(
+        maintenance_scheduler_module.asyncio, "get_running_loop", lambda: _FakeLoop()
+    )
+
+    await scheduler._perform_candidate_cleanup()
+
+    batch_one.query_obj.filter.return_value.order_by.return_value.limit.return_value.all.assert_called_once()
+    batch_one.query_obj.filter.return_value.order_by.return_value.limit.assert_called_once_with(2)
+    assert batch_one.committed is True
+    assert batch_one.closed is True
+    assert batch_two.closed is True