Are queries over Bolt visible in logs or other places and is it possible to secure them against unintended viewers who might intercept them?
Queries are visible inside logs in the case Memgraph is configured with the --log-level=DEBUG or --log-level=TRACE flags (by default, the log level is INFO -> queries are not visible). These files have to be secured on the OS level (unauthorized users shouldn't have access to the machine running Memgraph).
Memgraph supports secure Bolt connections (SSL connections), so the communication between clients and the Memgraph server can be encrypted (flags to achieve that are --bolt-key-file and --bolt-cert-file).