feat(selection): M6.3 选股结果落库 + /api/selections（快照 + 逐候选行）

- domain/repositories/selection.py + SelectionMeta：选股 Repository Protocol
- models/selection.py + migration a6c91d4e7f20：selection_snapshot（查询/统计快照）+
  selection_result（逐候选：rank/score/factor_values/filter_status/reason JSON）
- repositories/selection_impl.py：save/get/list_recent（读回重建 SelectionResult）
- api/selections.py：POST /api/selections（同步执行+落库，返回 selection_id+result）、
  GET /{id}、GET 列表（as_of/method 过滤）；deps 装配 SelectionService/Repo
- config._build_mysql_url：仅编码破坏 URL 结构的字符（修 Alembic configparser 遇 %21 崩）
- 迁移已在 MySQL 应用（alembic head a6c91d4e7f20）；tests/test_selections_api.py 4 例
  提交→读回一致/404/列表过滤/condition；全量 pytest 通过

backend/app/api/selections.py

@@ -0,0 +1,71 @@
+"""选股 API（M6.3）：提交/查询选股，结果落库可复现。
+
+POST /api/selections          同步执行一次选股并落库 → {selection_id, result}
+GET  /api/selections/{id}     读回某次选股完整结果
+GET  /api/selections          历史选股元数据（可过滤 as_of/method）
+
+同步执行：单日全市场因子评分/条件计算量轻（秒级）；未来若超时再迁 Job。
+"""
+
+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,
+    SelectionRepoDep,
+    SelectionServiceDep,
+)
+from app.application.services.job_executor import new_id
+from app.domain.entities.selection import SelectionMeta, SelectionQuery, SelectionResult
+
+router = APIRouter(prefix="/selections", tags=["selections"])
+
+
+class SelectionRun(BaseModel):
+    selection_id: str
+    result: SelectionResult
+
+
+@router.post("", response_model=SelectionRun, summary="执行一次选股（同步）并落库")
+def run_selection(
+    query: SelectionQuery,
+    service: SelectionServiceDep,
+    selection_repo: SelectionRepoDep,
+    session: DbSession,
+) -> SelectionRun:
+    result = service.select(query)
+    selection_id = new_id("SEL")
+    selection_repo.save(selection_id, result)
+    session.commit()
+    return SelectionRun(selection_id=selection_id, result=result)
+
+
+@router.get("/{selection_id}", response_model=SelectionResult, summary="读回一次选股结果")
+def get_selection(
+    selection_id: str,
+    selection_repo: SelectionRepoDep,
+) -> SelectionResult:
+    result = selection_repo.get(selection_id)
+    if result is None:
+        raise HTTPException(status_code=404, detail=f"选股记录 {selection_id} 不存在")
+    return result
+
+
+_AsOfQuery = Annotated[date | None, Query(description="按选股时点过滤")]
+_MethodQuery = Annotated[str | None, Query(pattern="^(score|condition)$")]
+_LimitQuery = Annotated[int, Query(ge=1, le=200)]
+
+
+@router.get("", response_model=list[SelectionMeta], summary="历史选股元数据列表")
+def list_selections(
+    selection_repo: SelectionRepoDep,
+    as_of: _AsOfQuery = None,
+    method: _MethodQuery = None,
+    limit: _LimitQuery = 20,
+) -> list[SelectionMeta]:
+    return selection_repo.list_recent(as_of=as_of, method=method, limit=limit)