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/infrastructure/persistence/sqlalchemy/models/__init__.py

@@ -30,3 +30,6 @@ from app.infrastructure.persistence.sqlalchemy.models.signal import (  # noqa: F
     SignalEventModel,
     SignalSnapshotModel,
 )
+from app.infrastructure.persistence.sqlalchemy.models.strategy import (  # noqa: F401
+    StrategyModel,
+)