Add embedded Express admin API and unified dev workflow.

Move admin backend into this repo (Express + MySQL on :8085) so npm run dev starts both API and Vite, with updated env defaults and Caddy config for gadm.tkmind.cn.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/db.mjs

@@ -0,0 +1,28 @@
+import mysql from 'mysql2/promise';
+
+export function isDatabaseConfigured() {
+  return Boolean(
+    process.env.DATABASE_URL ||
+      (process.env.MYSQL_HOST && process.env.MYSQL_DATABASE),
+  );
+}
+
+export function createDbPool() {
+  if (!isDatabaseConfigured()) {
+    throw new Error('MySQL 未配置，请在 .env 设置 DATABASE_URL 或 MYSQL_*');
+  }
+
+  if (process.env.DATABASE_URL) {
+    return mysql.createPool(process.env.DATABASE_URL);
+  }
+
+  return mysql.createPool({
+    host: process.env.MYSQL_HOST ?? 'localhost',
+    port: Number(process.env.MYSQL_PORT ?? 3306),
+    user: process.env.MYSQL_USER ?? 'boot',
+    password: process.env.MYSQL_PASSWORD ?? '',
+    database: process.env.MYSQL_DATABASE ?? 'tkmind',
+    waitForConnections: true,
+    connectionLimit: 10,
+  });
+}