feat(composite): M7.2b 因子组合落库 + /api/composites CRUD

- factor_composite 表（migration c3e9a0d1f4b5，MySQL 已应用；name 唯一）
- CompositeDefinition/Component 实体 + CompositeRepository Protocol + SQLAlchemy 实现
- /api/composites：POST（注册表自动填充组件 direction；未注册因子 400）、GET 列表/详情、DELETE
- tests/test_composites_api.py（repo CRUD/同名拒绝/删除、API 方向填充/404/400）；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.composite import CompositeRepository
 from app.domain.repositories.factor import FactorRepository
 from app.domain.repositories.jobs import ExperimentRepository, JobRepository
 from app.domain.repositories.market import (
@@ -19,6 +20,9 @@ from app.domain.repositories.market import (
     StockRepository,
 )
 from app.domain.repositories.selection import SelectionRepository
+from app.infrastructure.persistence.sqlalchemy.repositories.composite_impl import (
+    SqlAlchemyCompositeRepository,
+)
 from app.infrastructure.persistence.sqlalchemy.repositories.factor_impl import (
     SqlAlchemyFactorRepository,
 )
@@ -78,6 +82,10 @@ def _factor_repo_factory(session: DbSession) -> FactorRepository:
     return SqlAlchemyFactorRepository(session)
 
 
+def _composite_repo_factory(session: DbSession) -> CompositeRepository:
+    return SqlAlchemyCompositeRepository(session)
+
+
 StockRepoDep = Annotated[StockRepository, Depends(_stock_repo_factory)]
 DailyRepoDep = Annotated[DailyBarRepository, Depends(_daily_repo_factory)]
 EngineDep = Annotated[QuantEngine, Depends(_engine_factory)]
@@ -85,6 +93,7 @@ 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)]
+CompositeRepoDep = Annotated[CompositeRepository, Depends(_composite_repo_factory)]
 
 
 def _job_repo_factory(session: DbSession):