refactor: 更新迁移文件路径并重命名.gitkeep

- 更新迁移文件输出路径为 ./drizzle，替换原有路径 ./src/db/migrations
- 重命名迁移文件目录下的.gitkeep文件至drizzle目录下

drizzle.config.ts

@@ -2,8 +2,8 @@ import { defineConfig } from 'drizzle-kit'
 import { env } from '@/env'
 
 export default defineConfig({
+  out: './drizzle',
   schema: './src/db/schema/index.ts',
-  out: './src/db/migrations',
   dialect: 'postgresql',
   dbCredentials: {
     url: env.DATABASE_URL,