chore(vultr-vps): gitignore rendered backend.tf in ai-workspace env

backend.tf is a runtime artifact produced by render_backend_tf.py (contains the
S3 endpoint); keep it out of version control like the other rendered files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

terraform-hcl-standard/vultr-vps/envs/ai-workspace/.gitignore

@@ -7,6 +7,8 @@ cloud-init.yaml
 terraform.auto.tfvars.json
 cmdb.json
 inventory.ini
+# render_backend_tf.py 渲染的 S3 兼容 backend（含 endpoint，按环境派生，不入库）
+backend.tf
 
 # terraform 运行时
 .terraform/