feat(db): hydrogen 和 mileage 数据库连接支持 Docker 环境变量注入

mileage-db.ts 改为从环境变量读取，硬编码值作为 fallback；
docker-compose.yml 新增加氢站库和里程库的连接参数。

src/server/mileage-db.ts

@@ -1,11 +1,14 @@
 import mysql from 'mysql2/promise';
+import dotenv from 'dotenv';
+
+dotenv.config();
 
 const mileagePool = mysql.createPool({
-  host: '101.133.130.65',
-  port: 3306,
-  user: 'bi_reader_02',
-  password: 'bi_reader_02_Pass',
-  database: 'hydrogen_energy',
+  host: process.env.MILEAGE_DB_HOST || '101.133.130.65',
+  port: Number(process.env.MILEAGE_DB_PORT) || 3306,
+  user: process.env.MILEAGE_DB_USER || 'bi_reader_02',
+  password: process.env.MILEAGE_DB_PASSWORD || 'bi_reader_02_Pass',
+  database: process.env.MILEAGE_DB_NAME || 'hydrogen_energy',
   waitForConnections: true,
   connectionLimit: 5,
   queueLimit: 0,