feat(prisma): 添加 PostgreSQL 驱动适配器以支持连接池

使用 @prisma/adapter-pg 和 pg 包替换默认的 Prisma 客户端适配器
通过连接池管理数据库连接，提升并发性能和资源利用率

backend/package.json

@@ -16,15 +16,18 @@
   "dependencies": {
     "@elysiajs/cors": "^1.0.0",
     "@elysiajs/eden": "^1.0.0",
+    "@prisma/adapter-pg": "^7.4.2",
     "@prisma/client": "^7.4.2",
     "better-auth": "^1.5.4",
     "dotenv": "^17.3.1",
     "elysia": "^1.0.0",
+    "pg": "^8.20.0",
     "prisma": "^7.4.2"
   },
   "devDependencies": {
     "@prisma/config": "^7.4.2",
     "@types/bun": "latest",
+    "@types/pg": "^8.18.0",
     "eslint": "^10.0.3"
   }
 }