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/recovery_code.go

@@ -0,0 +1,49 @@
+package security
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+	"strings"
+	"unicode"
+)
+
+const RecoveryCodeCount = 10
+
+func GenerateRecoveryCodes(count int) ([]string, error) {
+	if count <= 0 {
+		count = RecoveryCodeCount
+	}
+	codes := make([]string, 0, count)
+	for i := 0; i < count; i++ {
+		raw := make([]byte, 8)
+		if _, err := rand.Read(raw); err != nil {
+			return nil, fmt.Errorf("generate recovery code: %w", err)
+		}
+		encoded := strings.ToUpper(hex.EncodeToString(raw))
+		codes = append(codes, encoded[0:4]+"-"+encoded[4:8]+"-"+encoded[8:12]+"-"+encoded[12:16])
+	}
+	return codes, nil
+}
+
+func NormalizeRecoveryCode(code string) string {
+	return strings.Map(func(r rune) rune {
+		if unicode.IsSpace(r) || r == '-' {
+			return -1
+		}
+		return unicode.ToUpper(r)
+	}, strings.TrimSpace(code))
+}
+
+func IsRecoveryCodeCandidate(code string) bool {
+	normalized := NormalizeRecoveryCode(code)
+	if len(normalized) != 16 {
+		return false
+	}
+	for _, r := range normalized {
+		if !('0' <= r && r <= '9') && !('A' <= r && r <= 'F') {
+			return false
+		}
+	}
+	return true
+}