Verify Downloaded Report

#1296 — check the report FILE you were handed (the JSON download).

Body: the document from GET /savings-reports/{id}?format=json. The signature is checked over badge-savings-report-v1 NUL + canonical JSON of its payload, so an edited figure, name or agent id fails. An integral number respelt by a JSON tool (0.0 -> 0) is not an edit (#1424); canonical_form says whether the current or the pre-#1424 form matched. A bad signature answers 200 with valid=false and no identity fields: an unauthenticated document is never echoed back as if it were trusted. Hostile bodies (oversize, bad UTF-8, too deep, duplicate keys) are 413/422, never 500. Public; flag off -> 404.