P2R-13b. POST /hosts/{id}/source-groups/{gid}/run accepts optional
bandwidth_up_kbps / bandwidth_down_kbps form fields, plumbs them onto
CommandRunPayload. Agent dispatcher already prefers per-job override
over host-wide caps (T1). UI wraps the Run-now button in a form with
a <details> 'Limit bandwidth for this run' disclosure containing two
KB/s inputs.
