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/migrations/versions/a6c91d4e7f20_selection_tables.py

@@ -0,0 +1,62 @@
+"""selection_snapshot / selection_result 表（M6.3 选股落库）
+
+Revision ID: a6c91d4e7f20
+Revises: d3f6c9a21b04
+Create Date: 2026-09-08
+
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "a6c91d4e7f20"
+down_revision: str | None = "d3f6c9a21b04"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "selection_snapshot",
+        sa.Column("id", sa.String(length=32), nullable=False),
+        sa.Column("as_of", sa.Date(), nullable=False),
+        sa.Column("method", sa.String(length=16), nullable=False),
+        sa.Column("query_json", sa.Text(), nullable=False),
+        sa.Column("statistics_json", sa.Text(), nullable=False),
+        sa.Column("created_at", sa.DateTime(), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index("ix_selection_snapshot_as_of", "selection_snapshot", ["as_of"])
+    op.create_index("ix_selection_snapshot_created_at", "selection_snapshot", ["created_at"])
+
+    op.create_table(
+        "selection_result",
+        sa.Column(
+            "id",
+            sa.BigInteger().with_variant(sa.Integer(), "sqlite"),
+            autoincrement=True,
+            nullable=False,
+        ),
+        sa.Column("selection_id", sa.String(length=32), nullable=False),
+        sa.Column("symbol", sa.String(length=12), nullable=False),
+        sa.Column("rank", sa.Integer(), nullable=False),
+        sa.Column("score", sa.Numeric(precision=14, scale=6), nullable=False),
+        sa.Column("factor_values_json", sa.Text(), nullable=True),
+        sa.Column("filter_status_json", sa.Text(), nullable=True),
+        sa.Column("reason_json", sa.Text(), nullable=True),
+        sa.Column("created_at", sa.DateTime(), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index("ix_selection_result_selection_id", "selection_result", ["selection_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_selection_result_selection_id", table_name="selection_result")
+    op.drop_table("selection_result")
+    op.drop_index("ix_selection_snapshot_created_at", table_name="selection_snapshot")
+    op.drop_index("ix_selection_snapshot_as_of", table_name="selection_snapshot")
+    op.drop_table("selection_snapshot")