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_provider_keys_key_command_service.py

@@ -287,13 +287,12 @@ def test_cleanup_key_references_preserves_usage_and_video_tasks(
     side_effects_module.cleanup_key_references(cast(Any, db), ["key-1", "key-2"])
 
     assert [(stmt.kind, stmt.model.__name__) for stmt in db.statements] == [
-        ("delete", "RequestCandidate"),
         ("delete", "GeminiFileMapping"),
         ("update", "Usage"),
         ("update", "VideoTask"),
     ]
-    assert db.statements[2].values_dict == {"provider_api_key_id": None}
-    assert db.statements[3].values_dict == {"key_id": None}
+    assert db.statements[1].values_dict == {"provider_api_key_id": None}
+    assert db.statements[2].values_dict == {"key_id": None}
 
 
 @pytest.mark.asyncio