Get Segment Pool Ladder

Pool ladder of one campaign: found -> verified -> ready -> contacted.

T-2751. The Leads tab showed a single raw lead count, so "found 20 000, sending stopped after 500" read as a bug: nothing said how many addresses passed verification, how many are still waiting their first email, and who already got which step of the sequence. Every step here is a subset of the one above it, and touches splits contacted by sequence step.

Loaded client-side next to /leads/breakdown (same reason: it must never block the tab's first paint). The two lead steps come from the trigger-maintained segment_lead_stats (T-1245) and from nowhere else: this endpoint never counts agent_lead itself. A campaign whose counter row is not published yet answers null for those steps — unknown, never zero — while the send-log steps answer as usual.