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/migrations.go

@@ -30,6 +30,7 @@ var migrations = []migration{
 	{7, "Benachrichtigungen", (*Store).migrateNotifications},
 	{8, "Journal", (*Store).migrateJournal},
 	{9, "Web-Push", (*Store).migratePush},
+	{10, "Registrierung und Einladungslinks", (*Store).migrateRegistration},
 }
 
 func (s *Store) migrate() error {