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/app/api/deps.py

@@ -11,6 +11,7 @@ from fastapi import Depends
 from sqlalchemy.orm import Session
 
 from app.application.services.selection_service import SelectionService
+from app.domain.repositories.factor import FactorRepository
 from app.domain.repositories.jobs import ExperimentRepository, JobRepository
 from app.domain.repositories.market import (
     DailyBarRepository,
@@ -18,6 +19,9 @@ from app.domain.repositories.market import (
     StockRepository,
 )
 from app.domain.repositories.selection import SelectionRepository
+from app.infrastructure.persistence.sqlalchemy.repositories.factor_impl import (
+    SqlAlchemyFactorRepository,
+)
 from app.infrastructure.persistence.sqlalchemy.repositories.market_impl import (
     SqlAlchemyDailyBarRepository,
     SqlAlchemyFinancialRepository,
@@ -70,12 +74,17 @@ def _selection_repo_factory(session: DbSession) -> SelectionRepository:
     return SqlAlchemySelectionRepository(session)
 
 
+def _factor_repo_factory(session: DbSession) -> FactorRepository:
+    return SqlAlchemyFactorRepository(session)
+
+
 StockRepoDep = Annotated[StockRepository, Depends(_stock_repo_factory)]
 DailyRepoDep = Annotated[DailyBarRepository, Depends(_daily_repo_factory)]
 EngineDep = Annotated[QuantEngine, Depends(_engine_factory)]
 ResearchServiceDep = Annotated[ResearchService, Depends(_service_factory)]
 SelectionServiceDep = Annotated[SelectionService, Depends(_selection_service_factory)]
 SelectionRepoDep = Annotated[SelectionRepository, Depends(_selection_repo_factory)]
+FactorRepoDep = Annotated[FactorRepository, Depends(_factor_repo_factory)]
 
 
 def _job_repo_factory(session: DbSession):