feat(db): SQLite 全量迁移至 MySQL（config.yaml 配置化 + 迁移脚本 + 一致性校验）

- config.yaml database.mysql：host/port/db/user/charset 明文可提交；密码经 password_env
  引用 .env 的 MYSQL_PASSWORD（AGENT.md §33 密钥不进 git）
- config.py _build_mysql_url：URL 优先级 DATABASE_URL env > database.mysql 段 > sqlite 兜底
- pyproject 引入 pymysql>=1.1
- tests/conftest.py 强制每进程 /tmp SQLite（测试绝不触 MySQL 开发库）；test_config 覆盖
  mysql 组装/密码可选/sqlite 兜底分支
- scripts/migrate_sqlite_to_mysql.py：sqlite→mysql 一次性迁移工具（keyset 分页 + chunk
  多值 INSERT + 攒批 commit + 幂等续传 + 低配 MySQL 节流 --throttle-sec + --verify-only
  行数与抽样一致性校验）；已用于 data/quant.db 约 1605 万行迁移并经校验一致

backend/uv.lock

@@ -3262,6 +3262,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/64/02/b2606ddf52fa615d4ff3edb2aa05fb064337fa871f99ebb184a39e051cc8/pymongo-4.18.0-cp314-cp314t-win_arm64.whl", hash = "sha256:4a81d166a43e8af1e5152b6854a263ba0a8831f7dd2ca1badc716f219f4f1bc0", size = 817605, upload-time = "2026-09-03T16:00:41.547Z" },
 ]
 
+[[package]]
+name = "pymysql"
+version = "1.2.0"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/c9/bc/1c6a92f385940f727daeecf3bacaf186e03875dff57197801046c583bcf0/pymysql-1.2.0.tar.gz", hash = "sha256:6c7b17ca686988104d7426c27895b455cdeea3e9d3ceb1270f0c3704fead8c33", size = 49021, upload-time = "2026-05-19T08:26:22.302Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/c4/bd/2534e130295c8cfd4f0a2e31623baab7502278f1e97bcfe61db75656a77f/pymysql-1.2.0-py3-none-any.whl", hash = "sha256:62169ce6d5510f08e140c5e7990ee884a9764024e4a9a27b2cc11f1099322ae0", size = 45716, upload-time = "2026-05-19T08:26:20.974Z" },
+]
+
 [[package]]
 name = "pyparsing"
 version = "3.3.2"
@@ -3540,6 +3549,7 @@ dependencies = [
     { name = "alembic" },
     { name = "fastapi" },
     { name = "httpx" },
+    { name = "pymysql" },
     { name = "pyqlib" },
     { name = "pyyaml" },
     { name = "sqlalchemy" },
@@ -3563,6 +3573,7 @@ requires-dist = [
     { name = "alembic", specifier = ">=1.13" },
     { name = "fastapi", specifier = ">=0.115" },
     { name = "httpx", specifier = ">=0.28.1" },
+    { name = "pymysql", specifier = ">=1.1" },
     { name = "pyqlib", git = "https://github.com/microsoft/qlib.git?rev=79633dd" },
     { name = "pyyaml", specifier = ">=6.0" },
     { name = "sqlalchemy", specifier = ">=2.0" },