refactor(db): 去掉 lazy singleton，改为模块级 const db

getDB/closeDB + 可空单例是 Cloudflare Workers 场景的模式——每个请求独
立上下文、不允许模块加载期副作用。在 Bun 单进程长驻服务下这些都是冗余
的仪式，徒增心智。

改为模块级 const db：
- src/server/db/index.ts 直接 export drizzle(...) 实例
- shutdown 插件用 db.$client.end() 收尾
- db.middleware.ts 跟随内部重命名以避免同名遮蔽（本身的去留放到下一
  次提交）

src/server/api/middlewares/db.middleware.ts

@@ -1,11 +1,11 @@
 import { os } from '@/server/api/server'
-import { getDB } from '@/server/db'
+import { db as dbSingleton } from '@/server/db'
 
-export const db = os.middleware(async ({ context, next }) => {
-  return next({
+export const db = os.middleware(async ({ context, next }) =>
+  next({
     context: {
       ...context,
-      db: getDB(),
+      db: dbSingleton,
     },
-  })
-})
+  }),
+)

src/server/db/index.ts

@@ -2,24 +2,9 @@ import { drizzle } from 'drizzle-orm/postgres-js'
 import { env } from '@/env'
 import * as schema from '@/server/db/schema'
 
-export const createDB = () =>
-  drizzle({
+export const db = drizzle({
   connection: env.DATABASE_URL,
   schema,
-  })
+})
 
-export type DB = ReturnType<typeof createDB>
-
-let _db: DB | null = null
-
-export const getDB = (): DB => {
-  _db ??= createDB()
-  return _db
-}
-
-export const closeDB = async () => {
-  if (_db) {
-    await _db.$client.end()
-    _db = null
-  }
-}
+export type DB = typeof db

src/server/plugins/shutdown.ts

@@ -1,4 +1,4 @@
-import { closeDB } from '@/server/db'
+import { db } from '@/server/db'
 
 export default () => {
   if (import.meta.dev) return
@@ -11,9 +11,8 @@ export default () => {
     }
     exiting = true
 
-    // Brief delay to let srvx close the HTTP server first
     setTimeout(async () => {
-      await closeDB()
+      await db.$client.end()
       process.exit(0)
     }, 500)
   }