feat(auth): invite links for several people, invite by email, registration with approval or email domains

- Invites: link /?invite=CODE, copy link, send by email, max uses with counter
- Registration modes closed | approval | domain | domain+approval | open, waiting list with approve/reject
- REGISTRATION_MODE / REGISTRATION_DOMAINS lock the admin setting; ALLOW_REGISTRATION=true is honoured

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/system.go

@@ -138,6 +138,10 @@ func (s *Store) GetSystemSettings() (*domain.SystemSettings, error) {
 				settings.SharingMode = v
 			case "allow_registration":
 				settings.AllowRegistration = (v == "true" || v == "1")
+			case "registration_mode":
+				settings.RegistrationMode = NormalizeRegistrationMode(v)
+			case "registration_domains":
+				settings.RegistrationDomains = v
 			case "instance_name":
 				settings.InstanceName = v
 			case "smtp_host":
@@ -173,10 +177,28 @@ func (s *Store) GetSystemSettings() (*domain.SystemSettings, error) {
 			}
 		}
 	}
+	// Older installations only knew the open/closed switch
+	if settings.RegistrationMode == "" {
+		settings.RegistrationMode = RegistrationClosed
+		if settings.AllowRegistration {
+			settings.RegistrationMode = RegistrationOpen
+		}
+	}
+	settings.AllowRegistration = settings.RegistrationMode == RegistrationOpen
 	return settings, nil
 }
 
 func (s *Store) UpdateSystemSettings(settings *domain.SystemSettings) error {
+	// Clients that only send the old switch keep working
+	settings.RegistrationMode = NormalizeRegistrationMode(settings.RegistrationMode)
+	if settings.RegistrationMode == "" {
+		settings.RegistrationMode = RegistrationClosed
+		if settings.AllowRegistration {
+			settings.RegistrationMode = RegistrationOpen
+		}
+	}
+	settings.AllowRegistration = settings.RegistrationMode == RegistrationOpen
+	settings.RegistrationDomains = strings.Join(ParseEmailDomains(settings.RegistrationDomains), ", ")
 	regStr := "false"
 	if settings.AllowRegistration {
 		regStr = "true"
@@ -221,7 +243,9 @@ func (s *Store) UpdateSystemSettings(settings *domain.SystemSettings) error {
 		('allow_private_webhooks', ?),
 		('backup_enabled', ?),
 		('backup_time', ?),
-		('backup_retention_days', ?)
+		('backup_retention_days', ?),
+		('registration_mode', ?),
+		('registration_domains', ?)
 		ON CONFLICT(key) DO UPDATE SET value = excluded.value`,
 		settings.SharingMode,
 		regStr,
@@ -238,6 +262,8 @@ func (s *Store) UpdateSystemSettings(settings *domain.SystemSettings) error {
 		strconv.FormatBool(settings.BackupEnabled),
 		settings.BackupTime,
 		strconv.Itoa(settings.BackupRetentionDays),
+		settings.RegistrationMode,
+		settings.RegistrationDomains,
 	)
 	return err
 }