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)

backend/app/infrastructure/persistence/sqlalchemy/repositories/composite_impl.py

@@ -0,0 +1,80 @@
+"""因子组合 Repository 的 SQLAlchemy 实现（M7.2b）。"""
+
+from __future__ import annotations
+
+import json
+from datetime import datetime
+
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from app.domain.entities.composite import CompositeComponent, CompositeDefinition
+from app.infrastructure.persistence.sqlalchemy.models.composite import FactorCompositeModel
+
+
+class SqlAlchemyCompositeRepository:
+    def __init__(self, session: Session) -> None:
+        self._session = session
+
+    def save(self, definition: CompositeDefinition) -> CompositeDefinition:
+        if not definition.id:
+            raise ValueError("需要 id（由调用方生成）")
+        exists = self._session.get(FactorCompositeModel, definition.id)
+        dup = self._session.scalar(
+            select(FactorCompositeModel)
+            .where(FactorCompositeModel.name == definition.name)
+            .limit(1)
+        )
+        if dup is not None and dup.id != definition.id:
+            raise ValueError(f"组合名已存在：{definition.name}")
+        now = definition.created_at or datetime.now()
+        if exists is None:
+            self._session.add(
+                FactorCompositeModel(
+                    id=definition.id,
+                    name=definition.name,
+                    method=definition.method,
+                    description=definition.description,
+                    components_json=json.dumps(
+                        [c.model_dump() for c in definition.components], ensure_ascii=False
+                    ),
+                    created_at=now,
+                )
+            )
+        else:
+            exists.name = definition.name
+            exists.method = definition.method
+            exists.description = definition.description
+            exists.components_json = json.dumps(
+                [c.model_dump() for c in definition.components], ensure_ascii=False
+            )
+        self._session.flush()
+        return definition
+
+    def get(self, composite_id: str) -> CompositeDefinition | None:
+        row = self._session.get(FactorCompositeModel, composite_id)
+        return _to_entity(row) if row else None
+
+    def list(self) -> list[CompositeDefinition]:
+        rows = self._session.scalars(
+            select(FactorCompositeModel).order_by(FactorCompositeModel.name)
+        ).all()
+        return [_to_entity(r) for r in rows]
+
+    def delete(self, composite_id: str) -> bool:
+        row = self._session.get(FactorCompositeModel, composite_id)
+        if row is None:
+            return False
+        self._session.delete(row)
+        return True
+
+
+def _to_entity(row: FactorCompositeModel) -> CompositeDefinition:
+    return CompositeDefinition(
+        id=row.id,
+        name=row.name,
+        method=row.method,
+        description=row.description,
+        components=[CompositeComponent(**c) for c in json.loads(row.components_json)],
+        created_at=row.created_at,
+    )