feat(factor): M7.1 因子定义入库 + /api/factors 读库（目录契约源）

- factor_definition 表（migration b7f2a5e81c33，MySQL 已应用）：name 主键 + 元数据
  （formula/brief/frequency/lookback/direction/requires JSON/version）+ FactorDefinition
  entity（from_registry_def 由代码注册表构造）
- FactorRepository Protocol + SQLAlchemy 实现（幂等 upsert/list/get）
- /api/factors 改读 DB；目录为空自动 seed 注册表（幂等）—— 保留自定义因子登记能力
  （计算仍须代码注册，引用未注册因子照常 FactorError，防伪因子）
- tests/test_factor_catalog.py（repo 幂等/roundtrip/registry seed、API seed+字段齐全）；
  test_api 的 client fixture 补 tmp sqlite session（factors 读库）；全量 pytest 通过

backend/app/infrastructure/persistence/migrations/versions/b7f2a5e81c33_factor_definition_table.py

@@ -0,0 +1,40 @@
+"""factor_definition 表（M7.1 因子定义入库）
+
+Revision ID: b7f2a5e81c33
+Revises: a6c91d4e7f20
+Create Date: 2026-09-09
+
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "b7f2a5e81c33"
+down_revision: str | None = "a6c91d4e7f20"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "factor_definition",
+        sa.Column("name", sa.String(length=64), nullable=False),
+        sa.Column("description", sa.String(length=500), nullable=False),
+        sa.Column("formula", sa.String(length=500), nullable=False),
+        sa.Column("brief", sa.String(length=500), nullable=False),
+        sa.Column("frequency", sa.String(length=16), nullable=False),
+        sa.Column("lookback", sa.Integer(), nullable=False),
+        sa.Column("direction", sa.String(length=32), nullable=False),
+        sa.Column("requires_json", sa.Text(), nullable=False),
+        sa.Column("version", sa.String(length=16), nullable=False),
+        sa.Column("created_at", sa.DateTime(), nullable=False),
+        sa.PrimaryKeyConstraint("name"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("factor_definition")