Adds a new tournament workflow step between draft and start:
- draft → "فتح التسجيل" → registration (players can join)
- registration → "بدء البطولة" → in_progress

New route: POST /tournaments/{id}/open-registration
Sets status from 'draft' to 'registration' with audit log.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>