feat(backend): add email validator package

Add ValidateEmail and NormalizeGiftCardCode functions with comprehensive tests. Migrate guest.go from inline mail.ParseAddress to validators.ValidateEmail.

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

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

backend/internal/validators/email.go

@@ -0,0 +1,40 @@
+package validators
+
+import (
+	"errors"
+	"net/mail"
+	"regexp"
+	"strings"
+)
+
+var emailRegex = regexp.MustCompile(`^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`)
+
+// ValidateEmail checks that an email is non-empty, within length limits,
+// and conforms to both RFC 5322 (via net/mail) and a basic regex shape.
+func ValidateEmail(email string) error {
+	email = strings.TrimSpace(email)
+	if email == "" {
+		return errors.New("email is required")
+	}
+	if len(email) > 254 {
+		return errors.New("email exceeds maximum length of 254 characters")
+	}
+	if _, err := mail.ParseAddress(email); err != nil {
+		return errors.New("invalid email format")
+	}
+	if !emailRegex.MatchString(email) {
+		return errors.New("invalid email format")
+	}
+	return nil
+}
+
+// NormalizeGiftCardCode strips non-alphanumeric characters and upper-cases the code.
+func NormalizeGiftCardCode(code string) string {
+	clean := ""
+	for _, char := range code {
+		if (char >= 'a' && char <= 'z') || (char >= 'A' && char <= 'Z') || (char >= '0' && char <= '9') {
+			clean += string(char)
+		}
+	}
+	return clean
+}

backend/internal/validators/email_test.go

@@ -0,0 +1,157 @@
+//go:build test
+// +build test
+
+package validators
+
+import (
+	"testing"
+)
+
+func TestValidateEmail_Valid(t *testing.T) {
+	valid := []string{
+		"user@example.com",
+		"user.name+tag@example.co.uk",
+		"a@b.cd",
+		"test@sub.example.org",
+		"123@example.com",
+	}
+	for _, e := range valid {
+		if err := ValidateEmail(e); err != nil {
+			t.Errorf("expected valid email %q, got error: %v", e, err)
+		}
+	}
+}
+
+func TestValidateEmail_Invalid(t *testing.T) {
+	invalid := []string{
+		"",
+		"not-an-email",
+		"@example.com",
+		"user@",
+		"user@.com",
+		"user@example",
+		"a@b.c", // TLD too short
+	}
+	for _, e := range invalid {
+		if err := ValidateEmail(e); err == nil {
+			t.Errorf("expected invalid email %q to return error", e)
+		}
+	}
+}
+
+func TestValidateEmail_TooLong(t *testing.T) {
+	long := string(make([]byte, 255))
+	if err := ValidateEmail(long); err == nil {
+		t.Error("expected error for email exceeding 254 chars")
+	}
+}
+
+func TestValidateEmail_WhitespaceTrimmed(t *testing.T) {
+	// ValidateEmail trims whitespace, so leading/trailing spaces are acceptable.
+	if err := ValidateEmail("  user@example.com  "); err != nil {
+		t.Errorf("expected valid after trimming whitespace, got: %v", err)
+	}
+}
+
+func TestNormalizeGiftCardCode_StripsNonAlphanumeric(t *testing.T) {
+	result := NormalizeGiftCardCode("abc-123_xyz!@#")
+	if result != "abc123xyz" {
+		t.Errorf("expected abc123xyz, got %s", result)
+	}
+}
+
+func TestNormalizeGiftCardCode_PreservesCase(t *testing.T) {
+	result := NormalizeGiftCardCode("aBcDeF123456")
+	if result != "aBcDeF123456" {
+		t.Errorf("expected aBcDeF123456, got %s", result)
+	}
+}
+
+func TestNormalizeGiftCardCode_Empty(t *testing.T) {
+	result := NormalizeGiftCardCode("")
+	if result != "" {
+		t.Errorf("expected empty string, got %s", result)
+	}
+}
+
+func TestNormalizeGiftCardCode_AlreadyClean(t *testing.T) {
+	result := NormalizeGiftCardCode("ABCDEF123456")
+	if result != "ABCDEF123456" {
+		t.Errorf("expected ABCDEF123456, got %s", result)
+	}
+}
+
+func TestValidateEmail_RejectsSQLInjection(t *testing.T) {
+	payloads := []string{
+		"' OR '1'='1",
+		"admin'--",
+		"'; DROP TABLE users;--",
+		`" OR 1=1 --`,
+		"' OR '1'='1' --",
+		"1' OR '1'='1",
+		"' UNION SELECT * FROM users --",
+		"admin'/*",
+	}
+	for _, p := range payloads {
+		if err := ValidateEmail(p); err == nil {
+			t.Errorf("expected SQLi payload %q to be rejected", p)
+		}
+	}
+}
+
+func TestValidateEmail_RejectsXSS(t *testing.T) {
+	payloads := []string{
+		"<script>alert(1)</script>",
+		"<img src=x onerror=alert(1)>",
+		"\"><script>alert(1)</script>",
+		"javascript:alert(1)",
+	}
+	for _, p := range payloads {
+		if err := ValidateEmail(p); err == nil {
+			t.Errorf("expected XSS payload %q to be rejected", p)
+		}
+	}
+}
+
+func TestValidateEmail_RejectsCommandInjection(t *testing.T) {
+	payloads := []string{
+		"; rm -rf /",
+		"| cat /etc/passwd",
+		"`id`",
+		"$(cat /etc/passwd)",
+	}
+	for _, p := range payloads {
+		if err := ValidateEmail(p); err == nil {
+			t.Errorf("expected command injection payload %q to be rejected", p)
+		}
+	}
+}
+
+func TestValidateEmail_RejectsControlChars(t *testing.T) {
+	payloads := []string{
+		"user@example.com\nX-Injected: header",
+		"user@example.com\r\nX-Injected: header",
+		"user\x00@example.com",
+		"user@ex\tample.com",
+	}
+	for _, p := range payloads {
+		if err := ValidateEmail(p); err == nil {
+			t.Errorf("expected control char payload %q to be rejected", p)
+		}
+	}
+}
+
+func TestValidateEmail_ValidMailsAreSafe(t *testing.T) {
+	// These are perfectly valid emails that happen to contain
+	// characters used in injection attacks — verify they pass.
+	valid := []string{
+		"safe.sql+select@example.com",
+		"safe.xss+script@example.co.uk",
+		"drop+table@example.org",
+	}
+	for _, e := range valid {
+		if err := ValidateEmail(e); err != nil {
+			t.Errorf("expected safe email %q to be valid, got: %v", e, err)
+		}
+	}
+}