Volume health for an inbox against its own baseline

Compares the requested window's message volume against this inbox's own recent norm and scores the deviation, for health/risk surfaces.

The baseline is a trailing average of the baselinePeriods windows of the same length immediately before dateFrom (default 3). No seasonal adjustment is applied. dateFrom and dateTo are both required; the window plus its baseline must stay within a 400-day scan, so long ranges need a lower baselinePeriods.

severityScore (0–100) tracks the size of a decline — growth scores 0 with label none. Bands: critical 75+, high 50–74, moderate 25–49, low 1–24.

trend compares the two halves of the requested window, which is what separates "down and still falling" from "down but recovering".

Insufficient history is reported explicitly. With no baseline volume, volumeVsBaselinePct and severityScore are null and severityLabel is insufficient_history — a silent -100% would be indistinguishable from a real collapse and would drive alerts the data can't justify.