feat(chart): M9-1 Chart DTO + Chart Service + Chart API（v3 §20）

- domain/entities/chart.py：ChartResult/OHLC/Volume/Series/EventMarker/ChartMetadata
  （adjust_mode + execution_price_basis 口径元数据）+ SelectionHit
- application/services/chart_service.py：个股 K线/量/MA 指标；显示层 qfq/hfq 折算
  （基于主口径 none 行情 × adjust_factor，绝回写研究数据）；回测个股视图把实际成交
  转 fills 标记并在显示口径不同时做坐标换算（v3 §20.3/§20.5）
- by-symbol 历史查询：SignalRepository/SelectionRepository.list_by_symbol（含溯源 id）
- api/charts.py：/stocks/{symbol}/chart|signals|selections、/backtests/{id}/stocks/{symbol}/chart
  |trades|positions
- tests/test_charts.py（指标/qfq-hfq 折算断言/回测 fills/API 集成+404）；全量 pytest 通过

backend/app/domain/entities/signal.py

@@ -55,3 +55,14 @@ class SignalMeta(BaseModel):
     watch: int = 0
     sell: int = 0
     created_at: datetime | None = None
+
+
+class SignalHit(BaseModel):
+    """个股在信号历史中的命中（by-symbol 查询，供 Chart 标记）。"""
+
+    signal_id: str
+    signal_date: date
+    signal_type: str
+    score: float | None = None
+    price: float | None = None
+    trigger_reason: list[str] = Field(default_factory=list)