DashboardAnalyticsController::calculateNetWorthAt ran one balance query per
account, and was called once per compared period, so the net-worth endpoint
issued ~2*N balance queries. It also re-fetched the account list on every call.
Fetch the accounts once and build a single BalanceLookup::forAccounts() over
the compared date range (the same batch loader already used by the balance
evolution endpoints), then read each balance from memory. Carry-forward
semantics match the previous 'latest balance on or before the date' query, so
results are unchanged; a query-count regression test locks the balance queries
to a fixed number regardless of account count.
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