feat(strategy): M8.3 策略模型 + /api/strategies（命名配置资产，可展开为 ResearchSpec）

- StrategyDefinition：universe/factors/selection/rebalance/costs/portfolio +
  price_adjustment（除 period 外完整策略定义）；to_research_spec(period) 展开为标准 Spec
- strategy 表（migration e1f2a3b4c5d6，MySQL 已应用；name 唯一）+ StrategyRepository
- /api/strategies：POST/GET/DELETE + POST /{id}/expand（period+initial_capital → ResearchSpec）
- tests/test_strategies.py（repo CRUD/同名/expand、API CRUD/400/404）；全量 pytest 通过

backend/tests/test_strategies.py

@@ -0,0 +1,123 @@
+"""M8.3 策略测试：repo CRUD + /api/strategies + expand 为 ResearchSpec。"""
+
+from __future__ import annotations
+
+from datetime import date
+
+import pytest
+from app.api import deps
+from app.domain.entities.research import SelectionSpec
+from app.domain.entities.strategy import StrategyDefinition
+from app.infrastructure.persistence.sqlalchemy.base import Base
+from app.infrastructure.persistence.sqlalchemy.repositories.strategy_impl import (
+    SqlAlchemyStrategyRepository,
+)
+from app.main import app
+from fastapi.testclient import TestClient
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+
+
+def _st() -> StrategyDefinition:
+    return StrategyDefinition(
+        name="质量成长动量",
+        description="ROE+动量（演示）",
+        factors=[{"name": "momentum_60", "weight": 1.0}],
+        selection=SelectionSpec(top_n=10),
+    )
+
+
+@pytest.fixture()
+def session(tmp_path):
+    engine = create_engine(f"sqlite:///{tmp_path / 'st.db'}", future=True)
+    Base.metadata.create_all(engine)
+    Session = sessionmaker(bind=engine, expire_on_commit=False)
+    with Session() as s:
+        yield s
+
+
+class TestStrategyRepository:
+    def test_save_get_list_delete(self, session) -> None:
+        repo = SqlAlchemyStrategyRepository(session)
+        repo.save(_st().model_copy(update={"id": "STG-T1"}))
+        session.commit()
+        got = repo.get("STG-T1")
+        assert got is not None and got.name == "质量成长动量"
+        assert got.selection.top_n == 10
+        assert len(repo.list()) == 1
+        assert repo.get_by_name("质量成长动量") is not None
+        assert repo.delete("STG-T1") is True
+        session.commit()
+        assert repo.get("STG-T1") is None
+
+    def test_duplicate_name(self, session) -> None:
+        repo = SqlAlchemyStrategyRepository(session)
+        repo.save(_st().model_copy(update={"id": "STG-A"}))
+        session.commit()
+        with pytest.raises(ValueError):
+            repo.save(_st().model_copy(update={"id": "STG-B"}))
+
+    def test_expand_to_research_spec(self, session) -> None:
+        st = _st().model_copy(update={"id": "STG-E"})
+        spec = st.to_research_spec((date(2024, 1, 1), date(2024, 6, 1)))
+        assert spec.type == "backtest"
+        assert spec.period == (date(2024, 1, 1), date(2024, 6, 1))
+        assert spec.factors[0].name == "momentum_60"
+        assert spec.price_adjustment == "none"
+
+
+@pytest.fixture()
+def client(tmp_path):
+    engine = create_engine(f"sqlite:///{tmp_path / 'api.db'}", future=True)
+    Base.metadata.create_all(engine)
+    Session = sessionmaker(bind=engine, expire_on_commit=False)
+
+    def _session_override():
+        with Session() as s:
+            yield s
+
+    app.dependency_overrides[deps.get_session] = _session_override
+    with TestClient(app) as c:
+        yield c
+    app.dependency_overrides.clear()
+
+
+class TestStrategiesApi:
+    def test_crud_and_expand(self, client) -> None:
+        body = {
+            "name": "演示策略",
+            "description": "动量",
+            "factors": [{"name": "momentum_60", "weight": 1}],
+            "selection": {"top_n": 10},
+        }
+        created = client.post("/api/strategies", json=body)
+        assert created.status_code == 200
+        sid = created.json()["id"]
+        assert sid.startswith("STG-")
+
+        assert len(client.get("/api/strategies").json()) == 1
+        detail = client.get(f"/api/strategies/{sid}").json()
+        assert detail["name"] == "演示策略"
+
+        resp = client.post(
+            f"/api/strategies/{sid}/expand",
+            json={"period": ["2024-01-01", "2024-06-01"]},
+        )
+        assert resp.status_code == 200
+        spec = resp.json()
+        assert spec["type"] == "backtest"
+        assert spec["factors"][0]["name"] == "momentum_60"
+
+        assert client.delete(f"/api/strategies/{sid}").status_code == 200
+        assert client.get(f"/api/strategies/{sid}").status_code == 404
+
+    def test_duplicate_and_bad_period(self, client) -> None:
+        body = {"name": "A", "factors": [{"name": "momentum_60", "weight": 1}]}
+        assert client.post("/api/strategies", json=body).status_code == 200
+        assert client.post("/api/strategies", json=body).status_code == 400
+        sid = client.get("/api/strategies").json()[0]["id"]
+        bad = client.post(
+            f"/api/strategies/{sid}/expand",
+            json={"period": ["2024-06-01", "2024-01-01"]},
+        )
+        assert bad.status_code == 400