feat(retention): 删除用户/Key 时保留历史记录，外键改 SET NULL 并添加名称快照

- Usage/RequestCandidate/VideoTask/Stats 等表的 user_id/api_key_id 外键从
  CASCADE 改为 SET NULL，删除用户或 Key 后历史记录不再丢失
- 各表添加 username/api_key_name 快照字段，删除后仍可追溯归属
- 新增 bulk_cleanup 模块，分批置空大表外键避免长事务锁
- 删除用户/Key 流程集成预清理步骤，先置空再删除
- 精简 candidate_builder 冗余 debug 日志
- 修复 proxy_nodes 启动日志 format 占位符错误（{} -> %s）
- 前端批量操作请求增加 5 分钟超时配置

tests/services/test_user_bulk_cleanup.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+from sqlalchemy import Column, Integer, String, create_engine
+from sqlalchemy.orm import Session, declarative_base, sessionmaker
+
+from src.services.user.bulk_cleanup import batch_nullify_fk
+
+Base = declarative_base()
+
+
+class DemoRow(Base):
+    __tablename__ = "demo_rows"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    ref_id = Column(String(36), nullable=True)
+
+
+def test_batch_nullify_fk_handles_sqlite_batches() -> None:
+    engine = create_engine("sqlite:///:memory:")
+    Base.metadata.create_all(engine)
+    session_factory = sessionmaker(bind=engine)
+    db: Session = session_factory()
+
+    try:
+        db.add_all([DemoRow(ref_id="entity-1") for _ in range(905)])
+        db.add_all([DemoRow(ref_id="entity-2") for _ in range(3)])
+        db.commit()
+
+        updated = batch_nullify_fk(db, DemoRow, "ref_id", "entity-1")
+
+        assert updated == 905
+        assert db.query(DemoRow).filter(DemoRow.ref_id.is_(None)).count() == 905
+        assert db.query(DemoRow).filter(DemoRow.ref_id == "entity-2").count() == 3
+    finally:
+        db.close()

tests/services/test_user_service_delete_user.py

@@ -1,11 +1,19 @@
 from __future__ import annotations
 
 from types import SimpleNamespace
-from unittest.mock import MagicMock
+from unittest.mock import AsyncMock, MagicMock, call
 
 import pytest
 
-from src.models.database import PaymentOrder, RefundRequest, User, Wallet
+from src.models.database import (
+    ApiKey,
+    PaymentOrder,
+    RefundRequest,
+    RequestCandidate,
+    Usage,
+    User,
+    Wallet,
+)
 from src.services.user.service import UserService
 
 
@@ -40,7 +48,7 @@ def test_delete_user_blocks_when_unfinished_refund_exists() -> None:
     db.query.side_effect = _query
 
     with pytest.raises(ValueError, match="未完结退款"):
-        UserService.delete_user.__wrapped__(db, "user-1")
+        UserService.delete_user(db, "user-1")
 
     db.delete.assert_not_called()
 
@@ -82,6 +90,68 @@ def test_delete_user_blocks_when_unfinished_payment_order_exists() -> None:
     db.query.side_effect = _query
 
     with pytest.raises(ValueError, match="未完结充值订单"):
-        UserService.delete_user.__wrapped__(db, "user-2")
+        UserService.delete_user(db, "user-2")
 
     db.delete.assert_not_called()
+
+
+def test_delete_user_precleans_large_tables_before_final_delete(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    user = SimpleNamespace(id="user-3", email="u3@example.com")
+
+    user_query = MagicMock()
+    user_query.filter.return_value = user_query
+    user_query.first.return_value = user
+
+    wallet_ids_query = MagicMock()
+    wallet_ids_query.outerjoin.return_value = wallet_ids_query
+    wallet_ids_query.filter.return_value = wallet_ids_query
+    wallet_ids_query.all.return_value = []
+
+    api_key_ids_query = MagicMock()
+    api_key_ids_query.filter.return_value = api_key_ids_query
+    api_key_ids_query.all.return_value = [("key-1",), ("key-2",)]
+
+    delete_query = MagicMock()
+    delete_query.filter.return_value = delete_query
+    delete_query.delete.return_value = 2
+
+    db = MagicMock()
+
+    def _query(model: object) -> MagicMock:
+        if model is User:
+            return user_query
+        if model is Wallet.id:
+            return wallet_ids_query
+        if model is ApiKey.id:
+            return api_key_ids_query
+        return delete_query
+
+    db.query.side_effect = _query
+
+    pre_clean_api_key = MagicMock()
+    batch_nullify_fk = MagicMock()
+    invalidate_user_cache = AsyncMock()
+    create_task = MagicMock()
+
+    monkeypatch.setattr("src.services.user.service.pre_clean_api_key", pre_clean_api_key)
+    monkeypatch.setattr("src.services.user.service.batch_nullify_fk", batch_nullify_fk)
+    monkeypatch.setattr(
+        "src.services.user.service.UserCacheService.invalidate_user_cache",
+        invalidate_user_cache,
+    )
+    monkeypatch.setattr("src.services.user.service.asyncio.create_task", create_task)
+
+    assert UserService.delete_user(db, "user-3") is True
+
+    assert pre_clean_api_key.call_args_list == [call(db, "key-1"), call(db, "key-2")]
+    assert batch_nullify_fk.call_args_list == [
+        call(db, Usage, "user_id", "user-3"),
+        call(db, RequestCandidate, "user_id", "user-3"),
+    ]
+    db.delete.assert_called_once_with(user)
+    db.commit.assert_called_once()
+    db.rollback.assert_not_called()
+    invalidate_user_cache.assert_called_once_with("user-3", "u3@example.com")
+    create_task.assert_called_once()