Kanban board summary: counts and totals of estimates grouped by estimate_status. Unlike Job's Kanban summary, Estimate's status groups are a fixed, hardcoded set (not per-category dynamic) — every active status is always present in the response, zero-filled if there are no matching estimates. AWAIT_SIGNATURE is omitted entirely when the company's multi-signer policy is disabled.

Accepts filter_rules/filter_rule_operator/preferred_timezone for scoping (same v3 rule-engine mechanism as the Estimate list endpoint). There is no dedicated is_proposal shortcut field here — filter on it via filter_rules instead (e.g. {"field": "is_proposal", "operator": "is", "value": true}).