feat(validation): add card number validation

backend/internal/card/validation.go

@@ -0,0 +1,27 @@
+package card
+
+import "errors"
+
+var (
+	ErrCardRequired = errors.New("card number is required")
+	ErrCardLength   = errors.New("card number must contain exactly 16 digits")
+	ErrCardNumeric  = errors.New("card number must contain only digits")
+)
+
+func Validate(number string) error {
+	if number == "" {
+		return ErrCardRequired
+	}
+
+	if len(number) != 16 {
+		return ErrCardLength
+	}
+
+	for _, char := range number {
+		if char < '0' || char > '9' {
+			return ErrCardNumeric
+		}
+	}
+
+	return nil
+}