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/infrastructure/persistence/sqlalchemy/models/selection.py

@@ -0,0 +1,42 @@
+"""选股持久化表（M6.3）。
+
+selection_snapshot：一次选股运行的查询与统计快照（复现/历史查询用）
+selection_result：逐候选行（symbol/rank/score + 可解释字段 JSON），
+回答 v2 §8「某日为什么选这只股票」。
+"""
+
+from __future__ import annotations
+
+from datetime import date, datetime
+
+from sqlalchemy import BigInteger, Date, DateTime, Integer, Numeric, String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.infrastructure.persistence.sqlalchemy.base import Base
+
+PK_INT = BigInteger().with_variant(Integer, "sqlite")
+
+
+class SelectionSnapshotModel(Base):
+    __tablename__ = "selection_snapshot"
+
+    id: Mapped[str] = mapped_column(String(32), primary_key=True)
+    as_of: Mapped[date] = mapped_column(Date, index=True)
+    method: Mapped[str] = mapped_column(String(16))
+    query_json: Mapped[str] = mapped_column(Text)
+    statistics_json: Mapped[str] = mapped_column(Text)
+    created_at: Mapped[datetime] = mapped_column(DateTime, index=True)
+
+
+class SelectionResultModel(Base):
+    __tablename__ = "selection_result"
+
+    id: Mapped[int] = mapped_column(PK_INT, primary_key=True, autoincrement=True)
+    selection_id: Mapped[str] = mapped_column(String(32), index=True)
+    symbol: Mapped[str] = mapped_column(String(12))
+    rank: Mapped[int] = mapped_column(Integer)
+    score: Mapped[float] = mapped_column(Numeric(14, 6))
+    factor_values_json: Mapped[str | None] = mapped_column(Text, nullable=True)
+    filter_status_json: Mapped[str | None] = mapped_column(Text, nullable=True)
+    reason_json: Mapped[str | None] = mapped_column(Text, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime)