- Add `generation_cooldown_seconds` column to `site_settings` (default 180s)
- Add `is_vip` column to `users` table (default 0/false)
- Introduce new `requireVip` middleware for enforcing VIP-only routes
- Restrict song generation access to VIP users or admins
- Add admin UI endpoints to grant/remove VIP status per user
- Allow admins to configure generation cooldown period via admin panel
- Update generation service to use dynamic cooldown (replacing hardcoded 3 minutes)
- Enhance generate page UI with Re-Gen button, improved placeholder text, and rate-limit state updates
- Update header navigation to show "My Music" and "Generate" only for VIP/admin users
