Queue an immediate sync for this tenant's active connection.

Answers accepted: false with a reason — never an error — when there is nothing for a sync to do: sync_in_progress while any of the connection's resources is still pending from an earlier dispatch, nothing_selected when the saved selection is narrowed to nothing. Both are decided here, before anything is spawned, because the dispatch itself runs detached: dispatch_sync would reach the same two verdicts (skip_if_pending and the selected == [] no-op), but as a SKIPPED nobody is awaiting, so a Sync-now button reading accepted: true would show a spinner over a sync that never started.