feat(backend): add payment discount preview and status tests

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/payments/refund_exclude_test.go

@@ -0,0 +1,110 @@
+//go:build test && dev
+// +build test,dev
+
+package payments
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"crussell/db"
+	"crussell/testutils/fixtures"
+)
+
+func setupRefundTestWithDiscount(t *testing.T) (string, string, float64) {
+	t.Helper()
+	userID, err := fixtures.CreateTestUser(db.DB)
+	if err != nil {
+		t.Fatalf("failed to create user: %v", err)
+	}
+	serviceID, err := fixtures.CreateTestService(db.DB)
+	if err != nil {
+		t.Fatalf("failed to create service: %v", err)
+	}
+	bookingID, err := fixtures.CreateTestBookingAtTime(db.DB, userID, serviceID, time.Date(2099, 12, 31, 10, 0, 0, 0, time.UTC))
+	if err != nil {
+		t.Fatalf("failed to create booking: %v", err)
+	}
+
+	// Insert a real cash payment of 50
+	_, err = db.DB.Exec(context.Background(), `
+		INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_at, updated_at)
+		VALUES ($1, 'full', 'cash', 5000, 'completed', NOW(), NOW())
+	`, bookingID)
+	if err != nil {
+		t.Fatalf("failed to create cash payment: %v", err)
+	}
+
+	// Insert a discount payment record (should be excluded from refund)
+	_, err = db.DB.Exec(context.Background(), `
+		INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_at, updated_at)
+		VALUES ($1, 'partial', 'discount', 500, 'completed', NOW(), NOW())
+	`, bookingID)
+	if err != nil {
+		t.Fatalf("failed to create discount payment: %v", err)
+	}
+
+	// Insert an on_the_house payment record (should also be excluded)
+	_, err = db.DB.Exec(context.Background(), `
+		INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_at, updated_at)
+		VALUES ($1, 'partial', 'on_the_house', 1000, 'completed', NOW(), NOW())
+	`, bookingID)
+	if err != nil {
+		t.Fatalf("failed to create on_the_house payment: %v", err)
+	}
+
+	return userID, bookingID, 50.0
+}
+
+func TestProcessCancellationRefund_ExcludesDiscountPayments(t *testing.T) {
+	resetTestData(t)
+	userID, bookingID, total := setupRefundTestWithDiscount(t)
+
+	farFuture := time.Date(2099, 12, 31, 10, 0, 0, 0, time.UTC)
+	now := farFuture.Add(-72 * time.Hour).Add(-1 * time.Hour) // >72h before
+
+	result, err := ProcessCancellationRefund(
+		context.Background(), bookingID, total, 50,
+		farFuture, now, "client_cancelled", &userID,
+	)
+	if err != nil {
+		t.Fatalf("ProcessCancellationRefund failed: %v", err)
+	}
+
+	// Refundable should be 50 (only the cash payment), not 65 (which would include discount + OTH)
+	if result.RefundableAmount != 50 {
+		t.Errorf("expected refundable 50 (excluding discount/OTH), got %.2f", result.RefundableAmount)
+	}
+}
+
+func TestProcessCancellationRefund_ExcludesOnTheHousePayments(t *testing.T) {
+	resetTestData(t)
+	userID, bookingID, total := setupRefundTestWithDiscount(t)
+
+	// Make on_the_house the only non-discount payment by marking the 50 cash as a payment that gets refunded
+	// but also add a pure on_the_house booking with no real money
+	_, err := db.DB.Exec(context.Background(), `
+		INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_at, updated_at)
+		VALUES ($1, 'partial', 'discount', 2500, 'completed', NOW(), NOW())
+	`, bookingID)
+	if err != nil {
+		t.Fatalf("failed to create extra discount: %v", err)
+	}
+
+	farFuture := time.Date(2099, 12, 31, 10, 0, 0, 0, time.UTC)
+	now := farFuture.Add(-72 * time.Hour).Add(-1 * time.Hour)
+
+	result, err := ProcessCancellationRefund(
+		context.Background(), bookingID, total, 50,
+		farFuture, now, "client_cancelled", &userID,
+	)
+	if err != nil {
+		t.Fatalf("ProcessCancellationRefund failed: %v", err)
+	}
+
+	// Refundable should be based on the actual cash payment only
+	if result.RefundableAmount != 50 {
+		t.Errorf("expected refundable 50 (real money only), got %.2f", result.RefundableAmount)
+	}
+}