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/internal/square/square_dev.go

@@ -23,12 +23,13 @@ func mockSleep(d time.Duration) {
 }
 
 type MockClient struct {
-	mu        sync.RWMutex
-	cards     map[string]map[string]*CardOnFile
-	checkouts map[string]*CheckoutResult
-	payments  map[string]*PaymentResult
-	refunds   map[string]*RefundResult
-	completed map[string]*PaymentResult
+	mu              sync.RWMutex
+	cards           map[string]map[string]*CardOnFile
+	checkouts       map[string]*CheckoutResult
+	payments        map[string]*PaymentResult
+	refunds         map[string]*RefundResult
+	completed       map[string]*PaymentResult
+	HoldCheckouts   bool
 }
 
 type devProdClient struct{}
@@ -117,41 +118,43 @@ func (m *MockClient) CreateCheckout(ctx context.Context, req CreateCheckoutReq)
 	m.checkouts[checkoutID] = result
 	m.mu.Unlock()
 
-	go func() {
-		defer func() {
-			if r := recover(); r != nil {
-				log.Printf("Panic recovered in Square mock payment processing: %v", r)
+	if !m.HoldCheckouts {
+		go func() {
+			defer func() {
+				if r := recover(); r != nil {
+					log.Printf("Panic recovered in Square mock payment processing: %v", r)
+				}
+			}()
+			mockSleep(3 * time.Second)
+
+			m.mu.Lock()
+			defer m.mu.Unlock()
+
+			paymentID := fmt.Sprintf("pay_%d", clock.Now().UnixNano())
+			amount := req.Amount
+			tipAmount := int64(0)
+			if req.TipEnabled {
+				tipAmount = 500
+				amount += tipAmount
 			}
+			fees := amount * 175 / 10000 // in-person rate: 1.75%
+
+			paymentResult := &PaymentResult{
+				ID:          paymentID,
+				Status:      "COMPLETED",
+				Amount:      amount,
+				CardBrand:   "VISA",
+				CardLast4:   "4242",
+				TipAmount:   tipAmount,
+				ReceiptURL:  "https://squareup.com/receipt/" + paymentID,
+				SquarePayID: "sqp_" + paymentID,
+				Fees:        fees,
+			}
+			m.completed[checkoutID] = paymentResult
+			m.checkouts[checkoutID].Status = "COMPLETED"
+			log.Printf("[SQUARE-MOCK] Checkout completed: id=%s, amount=%d, tip=%d", checkoutID, amount, tipAmount)
 		}()
-		mockSleep(3 * time.Second)
-
-		m.mu.Lock()
-		defer m.mu.Unlock()
-
-		paymentID := fmt.Sprintf("pay_%d", clock.Now().UnixNano())
-		amount := req.Amount
-		tipAmount := int64(0)
-		if req.TipEnabled {
-			tipAmount = 500
-			amount += tipAmount
-		}
-		fees := amount * 175 / 10000 // in-person rate: 1.75%
-
-		paymentResult := &PaymentResult{
-			ID:          paymentID,
-			Status:      "COMPLETED",
-			Amount:      amount,
-			CardBrand:   "VISA",
-			CardLast4:   "4242",
-			TipAmount:   tipAmount,
-			ReceiptURL:  "https://squareup.com/receipt/" + paymentID,
-			SquarePayID: "sqp_" + paymentID,
-			Fees:        fees,
-		}
-		m.completed[checkoutID] = paymentResult
-		m.checkouts[checkoutID].Status = "COMPLETED"
-		log.Printf("[SQUARE-MOCK] Checkout completed: id=%s, amount=%d, tip=%d", checkoutID, amount, tipAmount)
-	}()
+	}
 
 	return result, nil
 }
@@ -245,7 +248,7 @@ func (m *MockClient) CreateCardOnFileRaw(ctx context.Context, userID, cardNumber
 	brands := map[string]string{"4": "VISA", "5": "MASTERCARD", "3": "AMEX", "6": "DISCOVER"}
 	brand := brands[string(cardNumber[0])]
 	if brand == "" {
-		brand = "VISA"
+		brand = "UNKNOWN"
 	}
 
 	card := &CardOnFile{