With 5+ years of migrated history, loading every ligne up front took
~40s. The table now only ever holds a date-range window (~18 months
around today by default), extended by 6 months when scrolling near
either edge and trimmed from the far end past a 30-month cap. The
totals footer and "Année" filter can't be answered from a partial
window, so they're backed by two new small endpoints
(LedgerStatsController) instead: per-compte totals for whichever year
is currently scrolled into view, and the distinct list of years with
data.