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/infrastructure/persistence/sqlalchemy/models/__init__.py

@@ -4,6 +4,9 @@
 模型统一继承 infra.persistence.sqlalchemy.base.Base。
 """
 
+from app.infrastructure.persistence.sqlalchemy.models.composite import (  # noqa: F401
+    FactorCompositeModel,
+)
 from app.infrastructure.persistence.sqlalchemy.models.factor import (  # noqa: F401
     FactorDefinitionModel,
 )

backend/app/infrastructure/persistence/sqlalchemy/models/composite.py

@@ -0,0 +1,25 @@
+"""因子组合表（M7.2b）。
+
+factor_composite：可保存/复用的因子组合定义；components 以 JSON 存
+（组件方向冗余快照自因子注册表，计算时以注册表为准）。
+"""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlalchemy import DateTime, String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.infrastructure.persistence.sqlalchemy.base import Base
+
+
+class FactorCompositeModel(Base):
+    __tablename__ = "factor_composite"
+
+    id: Mapped[str] = mapped_column(String(32), primary_key=True)
+    name: Mapped[str] = mapped_column(String(64), unique=True)
+    method: Mapped[str] = mapped_column(String(16), default="fixed")
+    description: Mapped[str] = mapped_column(String(300), default="")
+    components_json: Mapped[str] = mapped_column(Text)
+    created_at: Mapped[datetime] = mapped_column(DateTime)