Refresh Lead Finder

Recompute the lead-finder read model now, instead of waiting for the tick.

Accepted, then done in the background — the work is cohort-sized and an HTTP request is the wrong place for it. Measured 27.08: a full pass over the 197k-lead probe outlived the gunicorn worker's max-requests recycle and came back as a 500 after 83 seconds, having done the work and thrown it away. The cron pass has no such ceiling; only this escape hatch did.

Poll GET …/lead-finder for the result: computed_at is the oldest stamp among the segments, so it moves when the pass has actually landed.