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/domain/repositories/strategy.py

@@ -0,0 +1,20 @@
+"""策略 Repository Protocol（M8.3）。"""
+
+from __future__ import annotations
+
+from typing import Protocol
+
+from app.domain.entities.strategy import StrategyDefinition
+
+
+class StrategyRepository(Protocol):
+    def save(self, definition: StrategyDefinition) -> StrategyDefinition:
+        """新建（name 冲突抛 ValueError）。"""
+
+    def get(self, strategy_id: str) -> StrategyDefinition | None: ...
+
+    def get_by_name(self, name: str) -> StrategyDefinition | None: ...
+
+    def list(self) -> list[StrategyDefinition]: ...
+
+    def delete(self, strategy_id: str) -> bool: ...