feat(signal): M8.1 交易信号引擎（规则 + signal_event 落库 + /api/signals）

- SignalRules（买入 rank 阈值/趋势 MA/动量 + 卖出区间/破位警示）+ SignalEvent
  （BUY/WATCH/SELL，score/price/trigger_reason 可解释）+ SignalResult/Meta
- quant/signal.generate_signals：与选股同一评分引擎取全市场 rank，按规则分类输出
- signal_snapshot/signal_event 表（migration d8e0b2f3c4d5，MySQL 已应用）+ Repo
- SignalService + POST /api/signals（同步+落库）、GET 详情/列表
- tests/test_signals.py（引擎分类/排序/破位不 BUY、service、API 提交读回）；全量 pytest 通过

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

@@ -26,3 +26,7 @@ from app.infrastructure.persistence.sqlalchemy.models.selection import (  # noqa
     SelectionResultModel,
     SelectionSnapshotModel,
 )
+from app.infrastructure.persistence.sqlalchemy.models.signal import (  # noqa: F401
+    SignalEventModel,
+    SignalSnapshotModel,
+)

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

@@ -0,0 +1,41 @@
+"""交易信号表（M8.1）。
+
+signal_snapshot：一次信号运行的查询/规则/统计快照
+signal_event：逐信号（signal_type + score/price + trigger_reason JSON）
+"""
+
+from __future__ import annotations
+
+from datetime import date, datetime
+
+from sqlalchemy import BigInteger, Date, DateTime, Integer, Numeric, String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.infrastructure.persistence.sqlalchemy.base import Base
+
+PK_INT = BigInteger().with_variant(Integer, "sqlite")
+
+
+class SignalSnapshotModel(Base):
+    __tablename__ = "signal_snapshot"
+
+    id: Mapped[str] = mapped_column(String(32), primary_key=True)
+    as_of: Mapped[date] = mapped_column(Date, index=True)
+    query_json: Mapped[str] = mapped_column(Text)
+    rules_json: Mapped[str] = mapped_column(Text)
+    statistics_json: Mapped[str] = mapped_column(Text)
+    created_at: Mapped[datetime] = mapped_column(DateTime, index=True)
+
+
+class SignalEventModel(Base):
+    __tablename__ = "signal_event"
+
+    id: Mapped[int] = mapped_column(PK_INT, primary_key=True, autoincrement=True)
+    signal_id: Mapped[str] = mapped_column(String(32), index=True)
+    symbol: Mapped[str] = mapped_column(String(12))
+    signal_date: Mapped[date] = mapped_column(Date)
+    signal_type: Mapped[str] = mapped_column(String(8))
+    score: Mapped[float | None] = mapped_column(Numeric(14, 6), nullable=True)
+    price: Mapped[float | None] = mapped_column(Numeric(14, 4), nullable=True)
+    reason_json: Mapped[str | None] = mapped_column(Text, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime)