chore: add backend/.env.example

backend/.env.example

@@ -0,0 +1,11 @@
+# Environment variables declared in this file are automatically made available to Prisma.
+# See the documentation for more detail: https://pris.ly/d/prisma-schema#accessing-environment-variables-from-the-schema
+
+# Prisma supports the native connection string format for PostgreSQL, MySQL, SQLite, SQL Server, MongoDB and CockroachDB.
+# See the documentation for all the connection string options: https://pris.ly/d/connection-strings
+
+# 数据库连接字符串（SQLite 示例）
+DATABASE_URL="file:./dev.db"
+
+# 后端服务端口（默认 3001）
+# PORT=3001