refactor: move yjs out of api into an app

[lihebi]

The yjs server was in apps/api folder, but it shares nothing with api server except prisma code. Since #480 extracted prisma into a package, this is a follow-up PR to move yjs code into apps/yjs.

These two servers are also conceptually separate:

• API server is stateless (thus can have multiple instances for horizontal scalability), while
• yjs server is stateful because peers must be connected to the same yjs server to sync in real-time.