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/domain/entities/signal.py

@@ -0,0 +1,57 @@
+"""交易信号领域实体（M8.1，v2 §15 Signal Engine）。
+
+Signal 输入 = Selection 排序（score）+ 价格/技术条件 + 规则；输出事件可解释：
+BUY / WATCH / SELL（破位警示），每条带 trigger_reason —— 回答
+「某日为什么对该股票给 BUY/SELL」（v2 §8）。
+"""
+
+from __future__ import annotations
+
+from datetime import date, datetime
+
+from pydantic import BaseModel, Field
+
+
+class SignalRules(BaseModel):
+    """规则（结构化，MVP）：买入区间 + 趋势/动量条件 + 卖出/警示区间。"""
+
+    buy_rank_threshold: int = Field(default=20, ge=1, le=500, description="rank<=此值进入买入候选")
+    buy_require_trend: bool = Field(default=True, description="买入需 close > MA(trend_ma)")
+    buy_require_momentum: bool = Field(default=False, description="买入需 close > 20 日前 close")
+    trend_ma: int = Field(default=60, ge=10, le=250)
+    sell_rank_threshold: int = Field(default=50, ge=1, le=1000, description="rank>此值或破位 → SELL 警示")
+    sell_on_trend_break: bool = Field(default=True, description="买入区间内 close < MA(trend_ma) → SELL")
+    max_output_rank: int = Field(default=80, ge=1, le=2000, description="仅输出排名前 N 的信号")
+
+
+class SignalEvent(BaseModel):
+    symbol: str
+    signal_date: date
+    signal_type: str = Field(pattern="^(BUY|WATCH|SELL)$")
+    score: float | None = None
+    price: float | None = None
+    trigger_reason: list[str] = Field(default_factory=list)
+
+
+class SignalStatistics(BaseModel):
+    universe_size: int = 0
+    buy: int = 0
+    watch: int = 0
+    sell: int = 0
+
+
+class SignalResult(BaseModel):
+    as_of_date: date
+    rules: SignalRules
+    statistics: SignalStatistics
+    events: list[SignalEvent] = Field(default_factory=list)
+    config_snapshot: dict = Field(default_factory=dict)
+
+
+class SignalMeta(BaseModel):
+    id: str
+    as_of: date
+    buy: int = 0
+    watch: int = 0
+    sell: int = 0
+    created_at: datetime | None = None

backend/app/domain/repositories/signal.py

@@ -0,0 +1,19 @@
+"""Signal Repository Protocol（M8.1 落库）。"""
+
+from __future__ import annotations
+
+from datetime import date
+from typing import Protocol
+
+from app.domain.entities.signal import SignalMeta, SignalResult
+
+
+class SignalRepository(Protocol):
+    def save(self, signal_id: str, result: SignalResult) -> None:
+        """snapshot 一行 + 事件逐行（同事务，调用方 commit）。"""
+
+    def get(self, signal_id: str) -> SignalResult | None: ...
+
+    def list_recent(
+        self, as_of: date | None = None, limit: int = 20
+    ) -> list[SignalMeta]: ...