refactor: 重构数据库实例创建逻辑并优化类型声明

- 重构数据库实例创建逻辑，将 `createDb` 函数改为箭头函数并优化 `getDb` 返回类型声明。

apps/server/src/db/index.ts

@@ -2,22 +2,21 @@ import { drizzle } from 'drizzle-orm/postgres-js'
 import * as schema from '@/db/schema'
 import { env } from '@/env'
 
-export function createDb() {
-  return drizzle({
+export const createDb = () =>
+  drizzle({
     connection: {
       url: env.DATABASE_URL,
       prepare: true,
     },
     schema,
   })
-}
 
 export type Db = ReturnType<typeof createDb>
 
 export const getDb = (() => {
   let db: Db | null = null
 
-  return (serverless: boolean = false) => {
+  return (serverless: boolean = false): Db => {
     if (serverless) {
       return createDb()
     }