chore(gitignore): 构建产物 - 忽略 backend/server 防止再次提交

.gitignore

@@ -73,6 +73,8 @@ camera_language_guide.md
 
 # Local backend executable; never commit build output.
 backend/cmd/server/server
+# 本项目附加：在 backend/ 目录直接 go build 会产出同名 server 二进制（曾误提交 62.7MB）。
+/backend/server
 # One-off local probe scripts and their compiled output.
 backend/tmp_*
 # Keep the runtime in source control; its optional debug map is not deployed.