Best Price min_distance now uses negative values (-50 to 0) to match
semantic meaning "below average". Peak Price continues using positive
values (0 to 50) for "above average".
Uniform formula: avg * (1 + distance/100) works for both period types.
Sign indicates direction: negative = toward MIN (cheap), positive = toward MAX (expensive).
Changes:
- const.py: DEFAULT_BEST_PRICE_MIN_DISTANCE_FROM_AVG = -5 (was 5)
- schemas.py: Best Price range -50 to 0, Peak Price range 0 to 50
- validators.py: Separate validate_best_price_distance_percentage()
- level_filtering.py: Simplified to uniform formula (removed conditionals)
- translations: Separate error messages for Best/Peak distance validation
- tests: 37 comprehensive validator tests (100% coverage)
Impact: Configuration UI now visually represents direction relative to average.
Users see intuitive negative values for "below average" pricing.
