backend: add optional q filter (name ILIKE or exact id) to the user
organisations list, add CountUserOrganisations for the total, and expose
total on the pagination payload.

frontend: iconified search affordance, x/y count in the header, debounced
typeahead, load-more kept inside the scroll area so the add-organisation
footer stays pinned.

tests: cover CountUserOrganisations for count, name ILIKE, exact id, and
soft-delete exclusion.