Audit and fix broken links in existing tool catalogue

## Business Goal
The tool catalogue is KI Hub's primary trust signal for developers and IT architects. Broken links undermine that trust immediately. Every broken link communicates: "this is not maintained." Fixing them before V2 launches is a prerequisite for credibility.

## Acceptance Criteria
- The user can click every link in the tool catalogue and reach a working destination
- The system has no entries with links returning 4xx or 5xx status
- Tool entries whose primary resource link cannot be fixed are either removed or marked as deprecated with a note explaining why

## Prerequisites
None — this is a standalone cleanup task.

## Dependencies
- Part of Epic 2 — AI Tools & Framework Catalogue (#39)

## Estimate
0.5 days