Add TypeScript types for handlers

**Problem**
When working with routes, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

Start by locating the route handlers and the existing error-handling entry points. Clarify whether the intended work is adding TypeScript types for handlers, improving route error messages, or both; completion should include the agreed type coverage and detailed, tested error messages.

Written by the indexing model from the issue text.