chore: add backend/uploads/ to gitignore and remove test data from git tracking

- Added backend/uploads/ to .gitignore
- Removed 171 test files from Git tracking (--cached, local files preserved)
  - Tool B: ~20 test files
  - Tool C: ~150 test files
- Local files are safe and not deleted
- This prevents future commits from tracking uploaded user files

.gitignore

@@ -48,6 +48,9 @@ tmp/
 temp/
 *.tmp
 
+# Backend uploads (用户上传的文件，不应提交到Git)
+backend/uploads/
+
 # ==================== Python ====================
 # Virtual environments (重要！避免提交 2+ GB 的依赖)
 venv/