feat(display): limit /adults and /juniors to last N months by default

Show only the last MONTHS_TO_SHOW months (default 5) in the fee table columns
so the page fits on screen without horizontal scrolling. Reconciliation still
runs over the full month history so balances, credits, and debts are unaffected.
Set MONTHS_TO_SHOW=0 to show all months. Implemented in both Python and Go.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

go/internal/web/api/handler.go

@@ -53,7 +53,7 @@ func (h *Handler) AssembleAdults(ctx context.Context) (AdultsResponse, error) {
 		return AdultsResponse{}, err
 	}
 	result := domreconcile.Reconcile(members, sortedMonths, txns, exceptions, time.Now().Year())
-	return buildAdultsResponse(members, sortedMonths, result, txns, h.Config, time.Now().Format("2006-01")), nil
+	return buildAdultsResponse(members, lastNMonths(sortedMonths, h.Config.MonthsToShow), result, txns, h.Config, time.Now().Format("2006-01")), nil
 }
 
 // ServeJuniors handles GET /api/juniors.
@@ -74,7 +74,16 @@ func (h *Handler) AssembleJuniors(ctx context.Context) (JuniorsResponse, error)
 		return JuniorsResponse{}, err
 	}
 	result := domreconcile.Reconcile(members, sortedMonths, txns, exceptions, time.Now().Year())
-	return buildJuniorsResponse(members, sortedMonths, result, txns, h.Config, time.Now().Format("2006-01")), nil
+	return buildJuniorsResponse(members, lastNMonths(sortedMonths, h.Config.MonthsToShow), result, txns, h.Config, time.Now().Format("2006-01")), nil
+}
+
+// lastNMonths returns the last n elements of months.
+// If n <= 0 or n >= len(months), the full slice is returned unchanged.
+func lastNMonths(months []string, n int) []string {
+	if n > 0 && len(months) > n {
+		return months[len(months)-n:]
+	}
+	return months
 }
 
 // ServePayments handles GET /api/payments.