feat(db): add gamification schema (avatars, frames, achievements, leagues)
- Add Avatar and Frame models for user customization
- Add Achievement and UserAchievement models
- Add League and LeagueParticipant models for competition
- Update User model with selectedAvatar and selectedFrame fields
- Update Enrollment model with achievement and league relations
- Add password support (passwordHash field in User model)
- Seed default avatars, frames, and demo tokens
Co-authored-by: Cursor cursoragent@cursor.com