Get the latest known balance for an account

Returns the bank aggregator's most recent balance snapshot for the account. This does not force the institution to recompute the balance — aggregators refresh on their own schedule (often daily, and for brokerage accounts only once the institution posts an update). Compare balanceAsOf (when the data was refreshed) with retrievedAt (when BankSync read it from the aggregator — the cached answer's time on a cache hit) to judge freshness; balanceAsOf is null when the provider does not report it, which means UNKNOWN rather than fresh. Responses may be served from a cache up to 6 hours old: X-BankSync-Cache says hit, miss or bypass, and Age is the cached answer's age in seconds.