Get product conversations by end user

Rank the verified end users of a product by widget conversations (or messages) started in the window. Conversations from visitors without a verified identity are reported as a separate unattributed total. Known limitation: a conversation linked to its end user only through the widget visitor (not stamped with the end user itself, as in product- or tenant-scoped surfaces) loses that link when the visitor is deleted after its idle TTL (default 90 days, configurable via visitorIdleDays). Such conversations then move from that end user to the unattributed total, so counts for older windows can drop over time.