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/router.py

@@ -8,12 +8,23 @@ from __future__ import annotations
 
 from fastapi import APIRouter
 
-from app.api import agent, experiments, factors, health, jobs, research, selections, stocks
+from app.api import (
+    agent,
+    composites,
+    experiments,
+    factors,
+    health,
+    jobs,
+    research,
+    selections,
+    stocks,
+)
 
 api_router = APIRouter()
 api_router.include_router(health.router)
 api_router.include_router(stocks.router)
 api_router.include_router(factors.router)
+api_router.include_router(composites.router)
 api_router.include_router(research.router)
 api_router.include_router(selections.router)
 api_router.include_router(jobs.router)