feat(factor): M7.1 因子定义入库 + /api/factors 读库（目录契约源）

- factor_definition 表（migration b7f2a5e81c33，MySQL 已应用）：name 主键 + 元数据
  （formula/brief/frequency/lookback/direction/requires JSON/version）+ FactorDefinition
  entity（from_registry_def 由代码注册表构造）
- FactorRepository Protocol + SQLAlchemy 实现（幂等 upsert/list/get）
- /api/factors 改读 DB；目录为空自动 seed 注册表（幂等）—— 保留自定义因子登记能力
  （计算仍须代码注册，引用未注册因子照常 FactorError，防伪因子）
- tests/test_factor_catalog.py（repo 幂等/roundtrip/registry seed、API seed+字段齐全）；
  test_api 的 client fixture 补 tmp sqlite session（factors 读库）；全量 pytest 通过

backend/tests/test_api.py

@@ -11,6 +11,7 @@ from datetime import date
 import pytest
 from app.api import deps
 from app.domain.entities.market import Stock
+from app.infrastructure.persistence.sqlalchemy.base import Base
 from app.main import app
 from app.quant.engine import LocalEngine
 from app.quant.service import ResearchService
@@ -41,7 +42,7 @@ def _mem_stocks() -> list[Stock]:
 
 
 @pytest.fixture()
-def client() -> TestClient:
+def client(tmp_path) -> TestClient:
     drifts = {sym: 0.003 - 0.0015 * i for i, sym in enumerate(_SYMS)}
     daily_df = synthetic_daily(drifts, n=300)
     bars = bars_dataframe_to_daily_bars(daily_df)
@@ -60,6 +61,20 @@ def client() -> TestClient:
     service = ResearchService(_MemStockRepo(_mem_stocks()), _MemDailyRepo(), LocalEngine())
     app.dependency_overrides[deps._stock_repo_factory] = lambda: _MemStockRepo(_mem_stocks())  # noqa: SLF001
     app.dependency_overrides[deps._service_factory] = lambda: service  # noqa: SLF001
+
+    # /api/factors 自 M7.1 读 DB（factor_definition）→ 提供 tmp sqlite session
+    from sqlalchemy import create_engine
+    from sqlalchemy.orm import sessionmaker
+
+    engine = create_engine(f"sqlite:///{tmp_path / 'api.db'}", future=True)
+    Base.metadata.create_all(engine)
+    SessionLocal = sessionmaker(bind=engine, expire_on_commit=False)
+
+    def _session_override():
+        with SessionLocal() as s:
+            yield s
+
+    app.dependency_overrides[deps.get_session] = _session_override
     with TestClient(app) as c:
         yield c
     app.dependency_overrides.clear()