fix: resolve RLock deadlock in Square mock client

Move m.mu.RUnlock to defer in GetCheckout and remove duplicate RLock/RUnlock around m.completed lookup. This prevents a deadlock when CheckoutPending error is returned but the RLock was already released.

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

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

backend/internal/square/square_dev.go

@@ -161,9 +161,9 @@ func (m *MockClient) GetCheckout(ctx context.Context, checkoutID string) (*Payme
 	log.Printf("[SQUARE-MOCK] GetCheckout: id=%s", checkoutID)
 
 	m.mu.RLock()
-	checkout, ok := m.checkouts[checkoutID]
-	m.mu.RUnlock()
+	defer m.mu.RUnlock()
 
+	checkout, ok := m.checkouts[checkoutID]
 	if !ok {
 		return nil, fmt.Errorf("checkout not found: %s", checkoutID)
 	}
@@ -172,10 +172,7 @@ func (m *MockClient) GetCheckout(ctx context.Context, checkoutID string) (*Payme
 		return nil, fmt.Errorf("checkout pending")
 	}
 
-	m.mu.RLock()
 	result, ok := m.completed[checkoutID]
-	m.mu.RUnlock()
-
 	if !ok {
 		return nil, fmt.Errorf("checkout result not found: %s", checkoutID)
 	}