feat(M01-B): 建立核心迁移与MySQL连接池

backend/src/db/mysql.ts

@@ -0,0 +1,23 @@
+import mysql, { type Pool, type PoolOptions } from 'mysql2/promise';
+import type { AppConfig } from '../config.js';
+
+export type MySqlPool = Pool;
+
+export function createMySqlPool(config: AppConfig): MySqlPool {
+  return mysql.createPool(toPoolOptions(config));
+}
+
+export function toPoolOptions(config: AppConfig): PoolOptions {
+  return {
+    host: config.mysql.host,
+    port: config.mysql.port,
+    database: config.mysql.database,
+    user: config.mysql.user,
+    password: config.mysql.password,
+    waitForConnections: true,
+    connectionLimit: config.mysql.connectionLimit,
+    namedPlaceholders: true,
+    timezone: 'Z',
+    dateStrings: false
+  };
+}