feat(ring-groups): allow direct voicemail redirection on no-answer failover

Add vm-<ext> destination support in UI destination picker, API validation, and [app-voicemail] Asterisk dialplan context. Refs #2

public/api.php

@@ -544,6 +544,14 @@ try {
             if ($name === '') {
                 throw new InvalidArgumentException("Le nom du groupe d'appel est obligatoire.");
             }
+            if (str_starts_with($failover, 'vm-')) {
+                $vmTarget = substr($failover, 3);
+                $vmStmt = $db->prepare('SELECT voicemail_enabled FROM extensions WHERE extension = :ext AND enabled = 1');
+                $vmStmt->execute([':ext' => $vmTarget]);
+                if (!(int)$vmStmt->fetchColumn()) {
+                    throw new InvalidArgumentException("Le répondeur du poste {$vmTarget} n'existe pas ou n'est pas activé.");
+                }
+            }
 
             if ($id) {
                 $stmt = $db->prepare('