Rework app configuration, validation, and image uploads

[Jorel97]

Closes #1

Summary

• make the server port and MySQL connection configurable through environment variables
• add JSON error helpers and required-field validation for signup, login, collection, photo, and upload routes
• harden /public/images/* serving against path traversal and return correct image content types
• make upload paths collision-resistant and sanitize collection directory names
• add focused Node test coverage for field validation, safe image resolution, content types, and upload path generation
• document local configuration and test commands

Verification

• node --check server.js
• node --check lib/http.js && node --check lib/static-images.js && node --check lib/uploads.js && node --check lib/fileserver.js
• node --test (8 passing)