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,
+)

backend/app/infrastructure/persistence/sqlalchemy/models/strategy.py

@@ -0,0 +1,22 @@
+"""strategy 表（M8.3）。"""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlalchemy import DateTime, String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.infrastructure.persistence.sqlalchemy.base import Base
+
+
+class StrategyModel(Base):
+    __tablename__ = "strategy"
+
+    id: Mapped[str] = mapped_column(String(32), primary_key=True)
+    name: Mapped[str] = mapped_column(String(64), unique=True)
+    description: Mapped[str] = mapped_column(String(300), default="")
+    spec_type: Mapped[str] = mapped_column(String(16), default="backtest")
+    config_json: Mapped[str] = mapped_column(Text)
+    version: Mapped[str] = mapped_column(String(16), default="1")
+    created_at: Mapped[datetime] = mapped_column(DateTime)