User Story: Handle/Log 400 Bad Request Errors

[givecoffee]

As a developer or backend user role, I want the API to validate the input and return descriptive 400 errors so that the frontend can guide users to fix the mistakes.

Criteria to fix this:

• Add input validation on ALL endpoints
• Return JSON with specific field errors
• Log the invalid requests with user context for debugging
• Add frontend validation to prevent the unnecessary error 400s