A job's log upload costs the same however long the log already is

services/actions/src/plan.rs job_report 'log' (about lines 2775-2825) runs SELECT SUM(LENGTH(text)) FROM logs WHERE job_id = ? on every log report to enforce MAX_LOG_BYTES (4 MB per job), so each report reads the whole log so far: the work grows quadratically over a chatty job. Batching in the runner (#336) keeps it to about one report a second, but a running byte count kept on the job (a migration) makes each report constant work.