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/tests/test_selections_api.py

@@ -0,0 +1,130 @@
+"""M6.3 选股 API 集成测试：POST /api/selections 落库 → GET 读回 → 历史列表。
+
+使用 tmp SQLite + 真实 SQLAlchemy Repository（override get_session），
+验证「提交→落库→读回一致」闭环与 v2 §8 历史选股查询。
+"""
+
+from __future__ import annotations
+
+from datetime import date
+
+import pytest
+from app.api import deps
+from app.domain.entities.market import Stock
+from app.infrastructure.persistence.sqlalchemy.base import Base
+from app.infrastructure.persistence.sqlalchemy.repositories.market_impl import (
+    SqlAlchemyDailyBarRepository,
+    SqlAlchemyStockRepository,
+)
+from app.main import app
+from fastapi.testclient import TestClient
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+
+from conftest_quant import bars_dataframe_to_daily_bars, synthetic_daily
+
+_SYMS = ["60000" + str(i) + ".SH" for i in range(5)]  # 600000~600004
+_AS_OF = date(2024, 12, 31)
+
+
+@pytest.fixture()
+def client(tmp_path) -> TestClient:
+    engine = create_engine(f"sqlite:///{tmp_path / 'api.db'}", future=True)
+    Base.metadata.create_all(engine)
+    SessionFactory = sessionmaker(bind=engine, expire_on_commit=False)
+
+    drifts = {s: 0.006 - 0.0015 * i for i, s in enumerate(_SYMS)}
+    daily_df = synthetic_daily(drifts, n=320)
+
+    with SessionFactory() as session:
+        stocks = [
+            Stock(symbol=s, name=f"测试股份{i}", industry="白酒",
+                  list_date=date(1999, 1, 1))
+            for i, s in enumerate(_SYMS)
+        ]
+        SqlAlchemyStockRepository(session).upsert_many(stocks)
+        bars = bars_dataframe_to_daily_bars(daily_df)
+        SqlAlchemyDailyBarRepository(session).upsert_many(bars)
+        session.commit()
+
+    def _session_override():
+        with SessionFactory() as session:
+            yield session
+
+    app.dependency_overrides[deps.get_session] = _session_override
+    with TestClient(app) as c:
+        yield c
+    app.dependency_overrides.clear()
+
+
+_SCORE_BODY = {
+    "universe": {"min_listing_days": 0},
+    "method": "score",
+    "factors": [{"name": "momentum_60", "weight": 1.0}],
+    "top_n": 3,
+    "as_of": _AS_OF.isoformat(),
+}
+
+
+class TestSelectionsApi:
+    def test_submit_then_read_back(self, client: TestClient) -> None:
+        resp = client.post("/api/selections", json=_SCORE_BODY)
+        assert resp.status_code == 200
+        body = resp.json()
+        sel_id = body["selection_id"]
+        assert sel_id.startswith("SEL-")
+        result = body["result"]
+        assert len(result["candidates"]) == 3
+        assert [c["rank"] for c in result["candidates"]] == [1, 2, 3]
+        assert result["candidates"][0]["factor_values"]  # 有因子值
+        assert result["candidates"][0]["selection_reason"]  # 可解释
+        assert result["as_of_date"] <= _AS_OF.isoformat()
+
+        # 读回一致
+        got = client.get(f"/api/selections/{sel_id}")
+        assert got.status_code == 200
+        g = got.json()
+        assert g["as_of_date"] == result["as_of_date"]
+        assert [c["symbol"] for c in g["candidates"]] == [
+            c["symbol"] for c in result["candidates"]
+        ]
+        assert g["statistics"]["selected"] == 3
+
+    def test_missing_id_404(self, client: TestClient) -> None:
+        assert client.get("/api/selections/SEL-NOPE").status_code == 404
+
+    def test_list_and_filter(self, client: TestClient) -> None:
+        client.post("/api/selections", json=_SCORE_BODY)
+        client.post(
+            "/api/selections",
+            json={**_SCORE_BODY, "top_n": 2, "method": "score",
+                  "factors": [{"name": "momentum_20", "weight": 1.0}]},
+        )
+        rows = client.get("/api/selections").json()
+        assert len(rows) >= 2
+        assert all(r["id"].startswith("SEL-") for r in rows)
+        assert all(r["method"] == "score" for r in rows)
+        assert all(r["selected"] > 0 for r in rows)
+        # as_of 过滤
+        by_date = client.get(f"/api/selections?as_of={_AS_OF.isoformat()}").json()
+        assert len(by_date) == len(rows)
+        empty = client.get("/api/selections?as_of=2020-01-01").json()
+        assert empty == []
+
+    def test_condition_submit(self, client: TestClient) -> None:
+        resp = client.post(
+            "/api/selections",
+            json={
+                "universe": {"min_listing_days": 0},
+                "method": "condition",
+                "conditions": [
+                    {"field": "static.industry", "op": "eq", "value": "白酒"},
+                    {"field": "momentum_60", "op": "gt", "value": 0},
+                ],
+                "as_of": _AS_OF.isoformat(),
+            },
+        )
+        assert resp.status_code == 200
+        body = resp.json()
+        assert len(body["result"]["candidates"]) == 5  # 全部上涨 → 5 只全过
+        assert all(c["filter_status"] for c in body["result"]["candidates"])