Keep the record count and total size in a totals table (closes #146)

info counted the records and added up their sizes with a query that
reads every record, which takes minutes on a large database. The
schema now has a totals table of one row, kept current by three
triggers on files, in the transaction of each insert, delete and size
update. info, scan's load phase and the read phases of report and
trees read that row instead of counting over files. scan writes files
as before.

Model: opus-5-5

scan.go

@@ -455,8 +455,8 @@ func (s *scanState) walkableRoots(roots []string, rules walkRules) []string {
 // included, so its display counts against the number of records.
 func (s *scanState) loadIndex(ctx context.Context, roots []string) error {
 	// Indexing tens of millions of records takes real time; without a
-	// display the scan looks hung before the walk begins. Counting them
-	// first gives the display its total.
+	// display the scan looks hung before the walk begins. The number of
+	// records, read first, gives the display its total.
 	prog := newProgress(progressParams{
 		label: scanPhase("load"), unit: scanUnit, total: -1,
 		waitingFor: countingRecords, stalledOn: "db",