feat(fees): update adult monthly rates for 2026-05 through 2026-08

- 2026-05: 700 → 450 CZK
- 2026-06, 07, 08: 600 CZK (new months)

Changes are mirrored in both Python (scripts/attendance.py) and Go (go/internal/domain/fees/fees.go).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go/internal/domain/fees/fees.go

@@ -11,7 +11,7 @@ const (
 var AdultFeeMonthlyRate = map[string]int{
 	"2025-09": 750, "2025-10": 750, "2025-11": 750, "2025-12": 750,
 	"2026-01": 750, "2026-02": 750, "2026-03": 350,
-	"2026-04": 700, "2026-05": 700,
+	"2026-04": 700, "2026-05": 450, "2026-06": 600, "2026-07": 600, "2026-08": 600,
 }
 
 // CalculateFee returns the adult fee in CZK for attendanceCount practices in