- New page /ventas to track characters selling items
- Default 12h duration per character sale
- Real-time countdown with color-coded badges (green/yellow/red/gray)
- Sound alert toggle when sales expire
- CRUD API: GET/POST/DELETE /api/sales
- Flyway migration V2 for character_sales table
- Private per-user (same user sees only their own characters)
- Expired sales remain visible with 'Expirado' badge until manually deleted
