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/api/signals.py

@@ -0,0 +1,65 @@
+"""交易信号 API（M8.1）：提交/查询信号（落库可复现）。
+
+POST /api/signals      body: {query: SelectionQuery, rules?: SignalRules}
+GET  /api/signals/{id} 读回某次信号
+GET  /api/signals      历史信号元数据（可过滤 as_of）
+"""
+
+from __future__ import annotations
+
+from datetime import date
+from typing import Annotated
+
+from fastapi import APIRouter, HTTPException, Query
+from pydantic import BaseModel
+
+from app.api.deps import DbSession, SignalRepoDep, SignalServiceDep
+from app.application.services.job_executor import new_id
+from app.domain.entities.selection import SelectionQuery
+from app.domain.entities.signal import SignalMeta, SignalResult, SignalRules
+
+router = APIRouter(prefix="/signals", tags=["signals"])
+
+_AsOfQuery = Annotated[date | None, Query(description="按信号时点过滤")]
+_LimitQuery = Annotated[int, Query(ge=1, le=200)]
+
+
+class SignalRequest(BaseModel):
+    query: SelectionQuery
+    rules: SignalRules = SignalRules()
+
+
+class SignalRun(BaseModel):
+    signal_id: str
+    result: SignalResult
+
+
+@router.post("", response_model=SignalRun, summary="生成一次交易信号（同步）并落库")
+def run_signal(
+    req: SignalRequest,
+    service: SignalServiceDep,
+    signal_repo: SignalRepoDep,
+    session: DbSession,
+) -> SignalRun:
+    result = service.signal(req.query, req.rules)
+    signal_id = new_id("SIG")
+    signal_repo.save(signal_id, result)
+    session.commit()
+    return SignalRun(signal_id=signal_id, result=result)
+
+
+@router.get("/{signal_id}", response_model=SignalResult, summary="读回一次信号结果")
+def get_signal(signal_id: str, signal_repo: SignalRepoDep) -> SignalResult:
+    result = signal_repo.get(signal_id)
+    if result is None:
+        raise HTTPException(status_code=404, detail=f"信号记录 {signal_id} 不存在")
+    return result
+
+
+@router.get("", response_model=list[SignalMeta], summary="历史信号元数据列表")
+def list_signals(
+    signal_repo: SignalRepoDep,
+    as_of: _AsOfQuery = None,
+    limit: _LimitQuery = 20,
+) -> list[SignalMeta]:
+    return signal_repo.list_recent(as_of=as_of, limit=limit)