fix: close documented platform polish gaps

api/src/channels/channels.service.ts

@@ -1417,8 +1417,8 @@ function normalizeExtensionDigits(value: unknown) {
     return 0;
   }
   const normalized = Number(value);
-  if (![0, 2, 4, 6].includes(normalized)) {
-    throw new BadRequestException('extensionDigits must be one of 0, 2, 4, or 6');
+  if (!Number.isInteger(normalized) || normalized < 0 || normalized > 20) {
+    throw new BadRequestException('extensionDigits must be an integer between 0 and 20');
   }
   return normalized;
 }