Replaces the O(N) list_sessions_rich histogram in /api/sessions/stats
with a single GROUP BY query, reducing response time from ~575ms to
<1ms on large databases.
Original PR #48921 by @liuhao1024. Salvage fixes based on review
feedback from teknium1 and @wernerhp:
1. Preserve try/except guard — a DB error still degrades to empty
by_source instead of failing the whole stats response.
2. GROUP BY COALESCE(source, 'cli') — the original GROUP BY source
could emit duplicate 'cli' keys (NULL group + literal 'cli' group)
that the dict comprehension silently dropped.
3. Add exclude_children=True — list_sessions_rich excludes subagent
runs, delegates, and compression continuations by default; the
bare GROUP BY counted all rows, inflating source counts.
Aggregate shape (exclude_children/include_archived/limit params)
adapted from closed duplicate #61120 by @mijanx.
Closes#48914
Co-authored-by: mijanx <mijanx@users.noreply.github.com>
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