refactor(db): 暴露 closeDB() 函数以支持连接池清理

src/server/db/index.ts

@@ -10,11 +10,16 @@ export const createDB = () =>
 
 export type DB = ReturnType<typeof createDB>
 
-export const getDB = (() => {
-  let db: DB | null = null
+let _db: DB | null = null
 
-  return (): DB => {
-    db ??= createDB()
-    return db
+export const getDB = (): DB => {
+  _db ??= createDB()
+  return _db
+}
+
+export const closeDB = async () => {
+  if (_db) {
+    await _db.$client.end()
+    _db = null
   }
-})()
+}