Groups by (bucket, source, api_key_id, api_key_name). Filters out
legacy by default. Returns both per-bucket detail and roll-ups
(by_source, by_key sorted desc and capped at 200, grand_total).
The MAX(created_at) projection is iterated via Rows().Scan into a
string column and parsed manually because the SQLite driver surfaces
the aggregated timestamp as a string, which database/sql refuses to
scan directly into time.Time. Postgres returns a real timestamp; the
same string path handles its RFC3339 form too.
Refs: #9862
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>