Rework app configuration and image serving

[Finesssee]

Closes #1

Summary

• make server port and MySQL connection settings configurable with environment variables
• harden /public/images/* serving with path traversal protection, file existence checks, and image content types
• validate required form/upload fields before writing records or files
• add collision-resistant upload filenames and a focused test suite for static image path handling
• document local development commands and runtime configuration

Verification

• npm test
• node --check server.js && node --check lib/fileserver.js && node --check lib/http.js && node --check lib/static-images.js
• git diff --check