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/domain/entities/composite.py

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+"""因子组合（Composite Factor）领域实体（M7.2b，v2 §13）。
+
+组合 = 一组 {因子, 权重} + method（MVP fixed：截面 zscore×方向×权重求和；
+方向在计算时取因子注册表元数据，落库时冗余快照以便列表展示）。
+"""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from pydantic import BaseModel, Field, model_validator
+
+
+class CompositeComponent(BaseModel):
+    name: str
+    weight: float = Field(default=1.0, gt=0)
+    direction: str = Field(default="higher_is_better")
+
+
+class CompositeDefinition(BaseModel):
+    id: str = ""
+    name: str = Field(min_length=1, max_length=64)
+    method: str = Field(default="fixed", pattern="^(fixed)$")
+    description: str = ""
+    components: list[CompositeComponent] = Field(min_length=1)
+    created_at: datetime | None = None
+
+    @model_validator(mode="after")
+    def _no_duplicate(self) -> CompositeDefinition:
+        names = [c.name for c in self.components]
+        if len(set(names)) != len(names):
+            raise ValueError("components 存在重复因子名")
+        return self

backend/app/domain/repositories/composite.py

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+"""Composite Factor Repository Protocol（M7.2b）。"""
+
+from __future__ import annotations
+
+from typing import Protocol
+
+from app.domain.entities.composite import CompositeDefinition
+
+
+class CompositeRepository(Protocol):
+    def save(self, definition: CompositeDefinition) -> CompositeDefinition:
+        """新建组合（name 冲突抛 ValueError）。"""
+
+    def get(self, composite_id: str) -> CompositeDefinition | None: ...
+
+    def list(self) -> list[CompositeDefinition]: ...
+
+    def delete(self, composite_id: str) -> bool:
+        """删除；不存在返回 False。"""