Add Category entity, edit event tabs (info/categories/stats/settings), CRUD categories
- Create Category entity: name, position (sortable), event, startAt, endAt, isActive()
- Default endAt: event.startAt - 1 day
- Add 4 tabs on edit event page: Informations, Categories/Billets, Statistiques, Parametres
- Add routes: add category, delete category, reorder categories (JSON API)
- Categories sorted by position, drag handle for future Sortable.js
- Active/Inactive badge based on date range
- Add migration for category table
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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