test: add coverage tests across backend + fix mock for PENDING checkout support

New test files cover previously untested paths across DAV, validators,
S3, Square, mw, bookings, user, and payments packages.

Includes mock fix: HoldCheckouts flag on MockClient allows tests to
pause auto-complete goroutine for testing PENDING checkout states.

Coverage: 50.4% → 65.0% (+14.6pp)

backend/handlers/payments/vat_test.go

@@ -4,6 +4,7 @@ package payments
 
 import (
 	"bytes"
+	"context"
 	"database/sql"
 	"encoding/json"
 	"net/http"
@@ -19,6 +20,7 @@ import (
 	"crussell/testutils/jwt"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestSPV_VATAppliedAtTillSale(t *testing.T) {
@@ -3520,3 +3522,11 @@ func TestVAT_DiscountAndCashPayment_RemainingBalance(t *testing.T) {
 		t.Errorf("expected RemainingAmount 60.00 (100.00 - 40.00), got %.2f", summary.RemainingAmount)
 	}
 }
+
+func TestGetVATConfig_QueryError(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	cfg, err := GetVATConfig(ctx, db.Conn)
+	assert.Error(t, err)
+	assert.Nil(t, cfg)
+}