After deleting a federated identity, the server-side user_federation_registry
and users.replicated_instances were not cleaned up, causing "already connected"
errors when trying to re-federate. The deletion endpoint now authoritatively
removes both the registry row and the replicatedInstances entry, and bumps the
LWW timestamp to prevent stale client syncs from re-inserting them.
Also extends the endpoint to accept mode 'leave' (skip S2S, just clean up),
and enables the "Select instances..." scope option in DeleteIdentityDialog.
Jannis Braun
02a44c201d