Build Puzzle Categories Module

[Mkalbani]

Description

Create a categories module that organises puzzles into thematic or skill-based groupings. Categories give players a sense of direction and allow the frontend to surface curated puzzle collections. They should be manageable by admins and assignable to puzzles at creation or edit time.

Acceptance Criteria

• A categories module is scaffolded at src/categories/
• Category entity includes id, name, slug, description, and createdAt
• POST /categories is restricted to admin users
• GET /categories returns all active categories with puzzle counts
• GET /categories/:slug/puzzles returns paginated puzzles within that category
• PATCH /categories/:id and DELETE /categories/:id are admin-only
• Slugs are auto-generated from name and must be unique
• Unit tests cover creation, slug generation, puzzle association, and deletion