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/migrations/versions/c3e9a0d1f4b5_factor_composite_table.py

@@ -0,0 +1,37 @@
+"""factor_composite 表（M7.2b 因子组合保存/复用）
+
+Revision ID: c3e9a0d1f4b5
+Revises: b7f2a5e81c33
+Create Date: 2026-09-09
+
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "c3e9a0d1f4b5"
+down_revision: str | None = "b7f2a5e81c33"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "factor_composite",
+        sa.Column("id", sa.String(length=32), nullable=False),
+        sa.Column("name", sa.String(length=64), nullable=False),
+        sa.Column("method", sa.String(length=16), nullable=False),
+        sa.Column("description", sa.String(length=300), nullable=False),
+        sa.Column("components_json", sa.Text(), nullable=False),
+        sa.Column("created_at", sa.DateTime(), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("name", name="uq_factor_composite_name"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("factor_composite")