♻️(backend) replace is_public with access_level field

Replace unused is_public boolean field with access_level to allow for more
granular control. Initially maintains public/restricted functionality while
enabling future addition of "trusted" access level.

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

@@ -2,8 +2,8 @@ export type ApiRoom = {
   id: string
   name: string
   slug: string
-  is_public: boolean
   is_administrable: boolean
+  access_level: string
   livekit?: {
     url: string
     room: string