🐛(backend) fix info panel crash for unregistered rooms

The info panel was crashing when opening a room that was not
registered in the database (with ALLOW_UNREGISTERED_ROOMS=true),
because the API response did not include the room slug.

Instead of adding frontend fallbacks, update the unregistered-room
response to include the slug, keeping the API contract consistent
with registered rooms.

Note: this still relies on the unregistered-room response staying
aligned with the registered-room schema. Any future divergence
between the two responses could introduce similar issues.
A refactoring on the backend side is needed.

It closes #1441.

src/frontend/src/features/rooms/api/ApiRoom.ts

@@ -22,7 +22,7 @@ export type ApiRoom = {
   id: string
   name: string
   slug: string
-  pin_code: string
+  pin_code?: string
   is_administrable: boolean
   access_level: ApiAccessLevel
   livekit?: ApiLiveKit