Get Summary Chart

Per-source × time-bucket series for the homepage "24h pulse" chart.

Reads pre-materialized 30-min rows from chart_buckets (written by app/pipeline/chart_bucket.py at the tail of each source's cycle), forward-fills missing buckets, then downsamples at read time for the 7d/30d windows.

Always returns one entry per source present in chart_buckets within the window. Sources with zero rows in the window are omitted (not zero-filled) — matches the prior behavior where a source with no event_snapshots produced no series row.