Fixed the backend’s dramatic flair: now invalid payloads get a proper…

[datagerl]

This change enables shared request validation for backend routes and standardizes the response shape for successful and failed requests.
Valid payloads now return a consistent success envelope with data and metadata.
Invalid payloads now return a predictable error envelope with clear details and error codes.
This improves API consistency and makes client-side handling more reliable.

closes #197

[Cedarich]

Hi @datagerl, thanks for working on this! I appreciate the effort to standardize request validation and response shapes. However, I noticed a significant architectural conflict:

The Issue:
Your PR adds a new standalone Node.js backend (package.json, server.js, src/app.js, src/validation.js) using the native http module. However, the repository already has an active NestJS TypeScript backend in /veilend-backend that serves as the primary API.

This creates:

• ❌ A duplicate backend that won't be maintained
• ❌ Conflicting response envelopes across the codebase
• ❌ Confusion about which backend to extend or deploy