feat(backend): update Square integration, validators, and image validation

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

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

backend/internal/square/square_dev_test.go

@@ -350,4 +350,4 @@ func TestDevClient_ConcurrentPayments(t *testing.T) {
 	if resultCount != 10 {
 		t.Errorf("expected 10 results, got %d", resultCount)
 	}
-}
+}