Get assistant statistics

Retrieve the "Statistiques" report for an assistant's inbound activity: total calls received with a 12-week activity chart, the weekday and hour distribution, and the duration breakdown of answered calls. The perimeter is invariant and matches the call-history page filtered on this agent and the inbound direction: calls with your organization id, this assistant id, direction 'inbound' and not soft-deleted — no status or date filter. All weekday/hour/week bucketing happens server-side in the assistant's reporting timezone (currently fixed to Europe/Paris). 'has_activity' is false when no inbound call exists yet (the UI then shows the empty state, never a zero-count test), and 'weekly_average' is null in that case. Durations cover answered calls only (last attempt with both answered_at and ended_at); the rest are counted as unanswered.