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>

src/models/database.py

@@ -2481,12 +2481,7 @@ class RequestCandidate(Base):
         nullable=True,
         index=True,
     )
-    key_id = Column(
-        String(36),
-        ForeignKey("provider_api_keys.id", ondelete="CASCADE"),
-        nullable=True,
-        index=True,
-    )
+    key_id = Column(String(36), nullable=True, index=True, comment="Provider Key ID 快照")
 
     # 状态信息
     status = Column(
@@ -2533,7 +2528,6 @@ class RequestCandidate(Base):
     api_key = relationship("ApiKey")
     provider = relationship("Provider")
     endpoint = relationship("ProviderEndpoint")
-    key = relationship("ProviderAPIKey")
 
 
 # ==================== 统计数据模型 ====================