Bulk-fetch observations for every series in a FRED release in one call. Use it to grab an entire economic report (jobs report, CPI report, GDP release) without enumerating constituent series IDs first. Returns paginated; pass lastSeriesId + lastObservationDate back to fetch the next page. ?releaseId=18 for H.15 (Selected Interest Rates), 10 for CPI, 50 for Employment Situation, 53 for GDP. Source: FRED API v2 /release/observations endpoint.