Build tooling improvements for 4.1.4

- Rename flash-pi.sh → flash-stock-img.sh for clarity
- Add 16GB partition sizing option (faster imaging)
- Disable Pi OS auto-expand to preserve partition size
- Add pip-audit security check to release validation
- Add config.json.example, gitignore actual config

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -84,3 +84,4 @@ pishrink.sh
 
 # Temp file storage
 frontends/web/temp_files/
+rpi/config.json