Returns an AI-generated summary of a WhatsApp group conversation for an exact date range. Note: the legacy /whatsapp/wa-group/ai-summary spelling still resolves via the generic group passthrough, but /channel/wa-group/ai-summary is the canonical path. The range is inclusive, interpreted in UTC at day granularity, and may span at most 7 days (Monday to Sunday is allowed; an 8th day is rejected). The summary is generated once per distinct date range and stored, so repeating the same request returns the stored copy without a further AI call. It is regenerated automatically when the conversation inside the range changes. Each generation consumes one AI summarisation credit; a stored result costs nothing. Requires the chat-summary feature to be enabled for the organization.