update_journey_entry now answers previous, the entry's earlier values of
exactly the fields it names (a suggestion written up goes back to being
one), and reorder_journey_entries answers previous_order, the ids in the
order they stood in before. Both come from the service the REST routes
use, which now builds an entry's patch in one pure helper; the routes'
bodies are unchanged. The tools' inline zod moves into strict schemas in
@trek/shared.
Maurice