Maity: Board

Hub: H3 follow-ups N4, N6, N7

tsk-88t5bec14qs3 proposed chore

From the #111 review: N4 cursor + batched lookups on GET placements?closed=true and a rate limit on status reports (one event per seq today); N6 new-profile handle clash only caught at pickup (H4); N7 a runner can burn its own seq to 2^31-1 (note at most). After H3 merges.