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/router.py

@@ -19,6 +19,7 @@ from app.api import (
     selections,
     signals,
     stocks,
+    strategies,
 )
 
 api_router = APIRouter()
@@ -29,6 +30,7 @@ api_router.include_router(composites.router)
 api_router.include_router(research.router)
 api_router.include_router(selections.router)
 api_router.include_router(signals.router)
+api_router.include_router(strategies.router)
 api_router.include_router(jobs.router)
 api_router.include_router(experiments.router)
 api_router.include_router(agent.router)