Add MCU prefix for MCU registrations

internal/repository/registration_prefix_test.go

@@ -0,0 +1,17 @@
+package repository
+
+import (
+	"testing"
+	"time"
+)
+
+func TestRegistrationIDPrefix(t *testing.T) {
+	registrationDate := time.Date(2026, time.September, 7, 0, 0, 0, 0, time.UTC)
+
+	if got, want := registrationIDPrefix(2, registrationDate), "MCU2609"; got != want {
+		t.Fatalf("MCU prefix = %q; want %q", got, want)
+	}
+	if got, want := registrationIDPrefix(1, registrationDate), "P2609"; got != want {
+		t.Fatalf("regular prefix = %q; want %q", got, want)
+	}
+}