feat(fees): adult rates for 2026-07 to 2026-12, default 600

- Default adult monthly rate 700 -> 600 CZK
- 2026-07 and 2026-08 set to 0; 2026-09 through 2026-12 added at 600
- Single-session fee is capped at the month's rate, so a 0-rate month
  is free for one session as well
- Mirrored in Go and legacy Python; new calculate_fee parity fixtures

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go/internal/domain/fees/fees.go

@@ -2,7 +2,7 @@
 package fees
 
 const (
-	AdultFeeDefault = 700 // CZK fallback for 2+ practices when month not in AdultFeeMonthlyRate
+	AdultFeeDefault = 600 // CZK fallback for 2+ practices when month not in AdultFeeMonthlyRate
 	AdultFeeSingle  = 200 // CZK for exactly 1 practice
 )
 
@@ -11,24 +11,27 @@ 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": 450, "2026-06": 600, "2026-07": 600, "2026-08": 600,
+	"2026-04": 700, "2026-05": 450, "2026-06": 600, "2026-07": 0, "2026-08": 0,
+	"2026-09": 600, "2026-10": 600, "2026-11": 600, "2026-12": 600,
 }
 
 // CalculateFee returns the adult fee in CZK for attendanceCount practices in
 // the given monthKey (format "YYYY-MM").
 //
 //	0 practices → 0
-//	1 practice  → AdultFeeSingle (200)
+//	1 practice  → AdultFeeSingle (200), capped at the month's rate so a
+//	              0-rate month is free for everyone
 //	2+          → AdultFeeMonthlyRate[monthKey] or AdultFeeDefault
 func CalculateFee(attendanceCount int, monthKey string) int {
 	if attendanceCount == 0 {
 		return 0
 	}
+	rate, ok := AdultFeeMonthlyRate[monthKey]
+	if !ok {
+		rate = AdultFeeDefault
+	}
 	if attendanceCount == 1 {
-		return AdultFeeSingle
+		return min(AdultFeeSingle, rate)
 	}
-	if rate, ok := AdultFeeMonthlyRate[monthKey]; ok {
-		return rate
-	}
-	return AdultFeeDefault
+	return rate
 }