feat(go/M2.3+M2.4): port domain/fees.CalculateFee and CalculateJuniorFee

Ports calculate_fee and calculate_junior_fee from scripts/attendance.py
into a new go/internal/domain/fees package. Introduces the Expected type
(Value int, Unknown bool) for the junior "?" sentinel, keeping the Go
API strictly typed instead of mirroring Python's str|int return.

All 20 table-driven tests pass with -race; golangci-lint clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/domain/fees/junior.go

@@ -0,0 +1,37 @@
+package fees
+
+const JuniorFeeDefault = 500 // CZK fallback for 2+ practices when month not in JuniorFeeMonthlyRate
+
+// JuniorFeeMonthlyRate mirrors JUNIOR_MONTHLY_RATE in scripts/attendance.py.
+// Months absent from this map fall back to JuniorFeeDefault.
+var JuniorFeeMonthlyRate = map[string]int{
+	"2025-09": 250,
+	"2026-03": 250,
+}
+
+// Expected is the result of a junior fee calculation.
+// When Unknown is true the fee requires manual review (Python returns "?");
+// in that case Value is meaningless — always check Unknown first.
+type Expected struct {
+	Value   int
+	Unknown bool
+}
+
+// CalculateJuniorFee returns the junior fee for attendanceCount practices in
+// the given monthKey (format "YYYY-MM").
+//
+//	0 practices → Expected{Value: 0}
+//	1 practice  → Expected{Unknown: true}  (manual review; Python sentinel "?")
+//	2+          → Expected{Value: JuniorFeeMonthlyRate[monthKey] or JuniorFeeDefault}
+func CalculateJuniorFee(attendanceCount int, monthKey string) Expected {
+	if attendanceCount == 0 {
+		return Expected{Value: 0}
+	}
+	if attendanceCount == 1 {
+		return Expected{Unknown: true}
+	}
+	if rate, ok := JuniorFeeMonthlyRate[monthKey]; ok {
+		return Expected{Value: rate}
+	}
+	return Expected{Value: JuniorFeeDefault}
+}