Make user creation asynchronous with a progress indicator

Is your feature request related to a problem? Please describe.
Creating a new user via the admin UI (or via occ user:add) can take several seconds to complete, depending on server configuration — active apps with user_created hooks (LDAP, Talk, GroupFolders, encryption key generation, etc.), background job mode (AJAX vs system cron), and database backend. During this time the UI simply appears frozen/loading with no feedback, which can make admins think the action failed or the server is unresponsive, especially when creating multiple users in sequence.

Describe the solution you'd like
Make user creation an asynchronous operation:

Return immediately after the core user record is created.
Run the remaining setup (encryption keys, app hooks, skeleton files, etc.) as a background job.
Show a progress indicator / status in the admin UI (e.g. "Setting up new account…") instead of a blocking spinner, ideally with a way to see when it's fully ready.

Describe alternatives you've considered

Documentation warning admins about expected delays (doesn't fix the UX issue).
Manually checking logs to confirm creation succeeded.

Additional context
This would improve UX especially for admins bulk-provisioning users, or on instances where certain apps (e.g. server-side encryption) are known to add noticeable latency to account creation.

Start by tracing the admin UI and occ user:add entry points to understand the current synchronous flow and where setup work runs. Define how background completion is reported to the UI, then verify that account creation returns promptly, setup status is visible, and the fully ready state can be recognized.