Open the database read-only for report and trees (closes #8)

report and trees now connect read-only (mode=ro, query_only, the same
busy timeout) and no longer set the journal mode, which is a write. A
read-only connection to a WAL database still needs its -wal and -shm
files, or write access to the directory to create them, so scan now
switches the database back to rollback-journal mode whenever it closes
it: between scans the file alone holds the database. If a report has
the database open at that moment the switch is refused; scan warns and
the database stays in WAL mode, with its -wal and -shm files, until the
next scan. README §Database states what readers need.

Model: opus-5-5

report.go

@@ -31,9 +31,9 @@ type scanRec struct {
 // loadRecords opens the database and reads every file record for the
 // report and trees subcommands. Any database problem — including a
 // missing database — is fatal. The error is returned rather than
-// exiting, so that the deferred close — which checkpoints the SQLite
-// WAL — always runs; the database is closed before the caller formats
-// its output, so it stays closed even if that output fails.
+// exiting, so that the deferred close always runs; the database is
+// closed before the caller formats its output, so it stays closed even
+// if that output fails.
 func loadRecords(ctx context.Context) ([]scanRec, error) {
 	dbPath := databasePath()