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()