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/app/api/strategies.py

@@ -0,0 +1,80 @@
+"""策略 API（M8.3）：/api/strategies CRUD + 展开为 ResearchSpec。
+
+POST /api/strategies   保存策略（name 唯一）
+GET  /api/strategies   列表
+GET  /api/strategies/{id}
+DELETE /api/strategies/{id}
+POST /api/strategies/{id}/expand  body: {period:[start,end], initial_capital?} → ResearchSpec
+"""
+
+from __future__ import annotations
+
+from datetime import date
+
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel, Field
+
+from app.api.deps import DbSession, StrategyRepoDep
+from app.application.services.job_executor import new_id
+from app.domain.entities.research import ResearchSpec
+from app.domain.entities.strategy import StrategyDefinition
+
+router = APIRouter(prefix="/strategies", tags=["strategies"])
+
+
+class ExpandRequest(BaseModel):
+    period: tuple[date, date]
+    initial_capital: float = Field(default=1_000_000.0, gt=0)
+
+
+@router.post("", response_model=StrategyDefinition, summary="保存策略")
+def create_strategy(
+    definition: StrategyDefinition,
+    strategy_repo: StrategyRepoDep,
+    session: DbSession,
+) -> StrategyDefinition:
+    try:
+        saved = strategy_repo.save(definition.model_copy(update={"id": new_id("STG")}))
+        session.commit()
+        return saved
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
+
+
+@router.get("", response_model=list[StrategyDefinition], summary="策略列表")
+def list_strategies(strategy_repo: StrategyRepoDep) -> list[StrategyDefinition]:
+    return strategy_repo.list()
+
+
+@router.get("/{strategy_id}", response_model=StrategyDefinition, summary="读取策略")
+def get_strategy(strategy_id: str, strategy_repo: StrategyRepoDep) -> StrategyDefinition:
+    row = strategy_repo.get(strategy_id)
+    if row is None:
+        raise HTTPException(status_code=404, detail=f"策略 {strategy_id} 不存在")
+    return row
+
+
+@router.delete("/{strategy_id}", summary="删除策略")
+def delete_strategy(
+    strategy_id: str,
+    strategy_repo: StrategyRepoDep,
+    session: DbSession,
+) -> dict:
+    if not strategy_repo.delete(strategy_id):
+        raise HTTPException(status_code=404, detail=f"策略 {strategy_id} 不存在")
+    session.commit()
+    return {"deleted": strategy_id}
+
+
+@router.post("/{strategy_id}/expand", response_model=ResearchSpec, summary="展开为研究 Spec")
+def expand_strategy(
+    strategy_id: str,
+    req: ExpandRequest,
+    strategy_repo: StrategyRepoDep,
+) -> ResearchSpec:
+    row = strategy_repo.get(strategy_id)
+    if row is None:
+        raise HTTPException(status_code=404, detail=f"策略 {strategy_id} 不存在")
+    if req.period[0] >= req.period[1]:
+        raise HTTPException(status_code=400, detail="period 必须满足 start < end")
+    return row.to_research_spec(period=req.period, initial_capital=req.initial_capital)