- Add `is_nsfw` column to `songs` and `playlists` tables
- Implement NSFW checkbox in song/playlist upload and edit forms
- Enforce NSFW content visibility: block unauthenticated users from viewing/streaming NSFW songs/playlists
- Filter NSFW content from public-facing lists (recent, popular, liked, etc.) unless user is logged in
- Add VIP-only visibility option for songs/playlists (admin/VIP users only)
- Display "EXPLICIT" badge on NSFW items in UI
- Add CSS styling for NSFW badge
- Update service layer to handle NSFW flag in CRUD operations and queries
