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/composites.py

@@ -0,0 +1,69 @@
+"""因子组合 API（M7.2b）：/api/composites CRUD。
+
+组合 = 可复用因子权重集；计算时方向取因子注册表，落库冗余快照。
+"""
+
+from __future__ import annotations
+
+from fastapi import APIRouter, HTTPException
+
+from app.api.deps import CompositeRepoDep, DbSession
+from app.application.services.job_executor import new_id
+from app.domain.entities.composite import CompositeComponent, CompositeDefinition
+from app.quant.factors import FactorError, get_factor
+
+router = APIRouter(prefix="/composites", tags=["composites"])
+
+
+def _fill_direction(definition: CompositeDefinition) -> CompositeDefinition:
+    """以因子注册表元数据补齐/校正组件 direction（登记但不可计算的因子报错）。"""
+    out = []
+    for c in definition.components:
+        try:
+            defn, _fn = get_factor(c.name)
+        except FactorError as exc:
+            raise HTTPException(status_code=400, detail=str(exc)) from exc
+        out.append(CompositeComponent(name=c.name, weight=c.weight, direction=defn.direction))
+    return definition.model_copy(update={"components": out})
+
+
+@router.post("", summary="保存因子组合", response_model=CompositeDefinition)
+def create_composite(
+    definition: CompositeDefinition,
+    composite_repo: CompositeRepoDep,
+    session: DbSession,
+) -> CompositeDefinition:
+    prepared = _fill_direction(definition.model_copy(update={"id": ""}))
+    try:
+        saved = composite_repo.save(
+            prepared.model_copy(update={"id": new_id("CF")})
+        )
+        session.commit()
+        return saved
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
+
+
+@router.get("", summary="因子组合列表", response_model=list[CompositeDefinition])
+def list_composites(composite_repo: CompositeRepoDep) -> list[CompositeDefinition]:
+    return composite_repo.list()
+
+
+@router.get("/{composite_id}", summary="读取因子组合", response_model=CompositeDefinition)
+def get_composite(composite_id: str, composite_repo: CompositeRepoDep) -> CompositeDefinition:
+    row = composite_repo.get(composite_id)
+    if row is None:
+        raise HTTPException(status_code=404, detail=f"组合 {composite_id} 不存在")
+    return row
+
+
+@router.delete("/{composite_id}", summary="删除因子组合")
+def delete_composite(
+    composite_id: str,
+    composite_repo: CompositeRepoDep,
+    session: DbSession,
+) -> dict:
+    if not composite_repo.delete(composite_id):
+        raise HTTPException(status_code=404, detail=f"组合 {composite_id} 不存在")
+    session.commit()
+    return {"deleted": composite_id}