Reimplement backend and frontend

[dzikoysk]

Reasons

• Spring performance and blocking IO
• Poorly documented OAuth2 API in Spring Security 2.4.x., lack of implementation for reactive WebFlux
• Legacy abstraction (backend + frontend)

Candidates

Backend:

• Reactive Spring: [Rejected] - Lack of OAuth 2.4.x implementation
• Javalin [Rejected] - Missing built-in OAuth support
• Ktor [Approved] - Non-blocking IO, lightweight, build-in OAuth support

Frontend:

• React - Next.js [Approved]
• Chakra UI [Approved]