docs: 添加项目结构规范文档

新增:
- docs/PROJECT_STRUCTURE.md - 完整目录结构规范
- data/.gitkeep, logs/.gitkeep, testdata/.gitkeep, uploads/avatars/.gitkeep

更新:
- .gitignore 添加临时文件规则 (*_result.txt, *_test*.txt 等)
- .gitignore 添加 uploads/avatars/ 内容忽略规则

.gitignore

@@ -60,3 +60,18 @@ node_modules/
 
 # NPM cache
 frontend/admin/.npm-cache/
+
+# Temporary output files
+*_result.txt
+*_test*.txt
+*_output.txt
+*_err.txt
+*.tmp
+*.temp
+
+# Uploads (keep directory but ignore contents)
+uploads/avatars/*
+!uploads/avatars/.gitkeep
+
+# Backup temp
+backup_temp/