feat(cli): add CLI for managing RSVPs and photo uploads

* implement runCLI function to handle subcommands
* add runRSVPCLI and runPhotosCLI for RSVP and photo management
* include usage instructions for CLI commands
* integrate CLI execution in main function

server/internal/mail/mail.go

@@ -49,7 +49,7 @@ func (m *Mailer) send(subject, body string) error {
 	)
 
 	recipients := []string{m.cfg.To}
-	for _, bcc := range strings.Split(m.cfg.Bcc, ",") {
+	for bcc := range strings.SplitSeq(m.cfg.Bcc, ",") {
 		if bcc = strings.TrimSpace(bcc); bcc != "" {
 			recipients = append(recipients, bcc)
 		}