feat: add complete MFA support

Add complete MFA support with TOTP, recovery codes, WebAuthn, trusted-device cookie flow, and email/SMS OTP delivery via notification channels. Security follow-up: trusted device tokens are stored in HttpOnly cookies, and SMS OTP reuses the existing Webhook notifier to avoid introducing a new dynamic URL sink.

server/internal/security/totp.go

@@ -0,0 +1,68 @@
+package security
+
+import (
+	"bytes"
+	"encoding/base64"
+	"image/png"
+	"strings"
+	"time"
+	"unicode"
+
+	"github.com/pquerna/otp"
+	"github.com/pquerna/otp/totp"
+)
+
+const TOTPIssuer = "BackupX"
+
+type TOTPEnrollment struct {
+	Secret        string
+	OTPAuthURL    string
+	QRCodeDataURL string
+}
+
+func GenerateTOTPEnrollment(accountName string) (*TOTPEnrollment, error) {
+	key, err := totp.Generate(totp.GenerateOpts{
+		Issuer:      TOTPIssuer,
+		AccountName: accountName,
+		Period:      30,
+		SecretSize:  20,
+		Digits:      otp.DigitsSix,
+		Algorithm:   otp.AlgorithmSHA1,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	image, err := key.Image(220, 220)
+	if err != nil {
+		return nil, err
+	}
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, image); err != nil {
+		return nil, err
+	}
+
+	return &TOTPEnrollment{
+		Secret:        key.Secret(),
+		OTPAuthURL:    key.URL(),
+		QRCodeDataURL: "data:image/png;base64," + base64.StdEncoding.EncodeToString(buf.Bytes()),
+	}, nil
+}
+
+func ValidateTOTPCode(secret string, code string) (bool, error) {
+	return totp.ValidateCustom(NormalizeTOTPCode(code), secret, time.Now().UTC(), totp.ValidateOpts{
+		Period:    30,
+		Skew:      1,
+		Digits:    otp.DigitsSix,
+		Algorithm: otp.AlgorithmSHA1,
+	})
+}
+
+func NormalizeTOTPCode(code string) string {
+	return strings.Map(func(r rune) rune {
+		if unicode.IsSpace(r) {
+			return -1
+		}
+		return r
+	}, strings.TrimSpace(code))
+}