Get inbox response-time stats

Time-to-first-response stats. Pairs each received message with the next sent message in the same conversation and reports the delta as both summary statistics and a fixed-bucket histogram suited for the analytics page's TTR chart.

sampleSize reflects only conversations that received AND got a reply in the window — received-but-never-answered conversations are excluded. Compare against /v1/analytics/inbox/volume's summary.received to compute reply rate.

Max date range is 365 days.