feat(fees): junior rate for 2026-08 is 0

- 2026-08 added to the junior rate table at 0 CZK, same as adults
- A single practice in a 0-rate month is 0 instead of "?", since a
  free month needs no manual review
- Mirrored in Go and legacy Python; new calculate_junior_fee parity
  fixtures

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

go/internal/domain/fees/junior.go

@@ -7,6 +7,7 @@ const JuniorFeeDefault = 500 // CZK fallback for 2+ practices when month not in
 var JuniorFeeMonthlyRate = map[string]int{
 	"2025-09": 250,
 	"2026-03": 250,
+	"2026-08": 0,
 }
 
 // Expected is the result of a junior fee calculation.
@@ -21,17 +22,19 @@ type Expected struct {
 // the given monthKey (format "YYYY-MM").
 //
 //	0 practices → Expected{Value: 0}
-//	1 practice  → Expected{Unknown: true}  (manual review; Python sentinel "?")
+//	1 practice  → Expected{Unknown: true}  (manual review; Python sentinel "?"),
+//	              or Expected{Value: 0} when the month's rate is 0
 //	2+          → Expected{Value: JuniorFeeMonthlyRate[monthKey] or JuniorFeeDefault}
 func CalculateJuniorFee(attendanceCount int, monthKey string) Expected {
 	if attendanceCount == 0 {
 		return Expected{Value: 0}
 	}
-	if attendanceCount == 1 {
+	rate, ok := JuniorFeeMonthlyRate[monthKey]
+	if !ok {
+		rate = JuniorFeeDefault
+	}
+	if attendanceCount == 1 && rate > 0 {
 		return Expected{Unknown: true}
 	}
-	if rate, ok := JuniorFeeMonthlyRate[monthKey]; ok {
-		return Expected{Value: rate}
-	}
-	return Expected{Value: JuniorFeeDefault}
+	return Expected{Value: rate}
 }