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

scan.go

@@ -87,8 +87,9 @@ type fileMeta struct {
 // hash only when its size, head, and tail match another file's. Flag
 // parsing and the at-least-one-operand check are done by cobra. Errors
 // are returned rather than exiting, so that the deferred close — which
-// checkpoints the SQLite WAL — always runs. Cancelling ctx unwinds the
-// worker pools and aborts the scan with the context's error.
+// takes the database out of WAL mode — always runs. Cancelling ctx
+// unwinds the worker pools and aborts the scan with the context's
+// error.
 func runScan(ctx context.Context, roots []string, workers int,
 	oneFS bool,
 ) error {
@@ -108,7 +109,7 @@ func runScan(ctx context.Context, roots []string, workers int,
 		return err
 	}
 
-	defer func() { _ = db.Close() }()
+	defer closeScanDatabase(ctx, db, dbPath)
 
 	st, err := syncScan(ctx, db, roots, workers, oneFS)
 	if err != nil {