Add support for re-enabling disabled managed users by organization owners

[Anty0]

Following the merge of PR #3263, we currently support disabling managed users. However, the system does not yet provide a clear way for organization owners to re-enable disabled users.

Solution:

• Add dedicated endpoints to disable and enable managed accounts by organization owners (instead of convoluting the leave endpoint as was done as a hot fix in the last PR – revert that)
• Keep disabled users visible in the organization member list but visually distinguish them (e.g., gray them out).
• Add UI controls to enable/disable users for organization owners if the organization manages them.
• Add tests for disabling and re-enabling managed users.