Implements gym training logic with daily session tracking:
- GymSession model manages training sessions per stat per day
- TrainingService handles training validation, energy consumption, and stat gains
- Lazy daily reset: session count resets when reset_date is before today
- Validates: sufficient energy, daily cap (5 sessions per stat), valid stats
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
