The account-deletion reconcile in account.ts uses a correlated subquery
to pick the latest-per-event row from memory_completions. Without an
index whose leading column is event_id the inner subquery falls back to
scanning sqlite_autoindex_memory_completions_1, making the reconcile
O(rows^2) — observed cost in production was ~171k rows read for one
deletion against ~407 rows. With this covering index the inner becomes
a seek and the reconcile drops to ~rows*log(rows).
