`spacetime logs` and `/v1/database/:name/logs` only retrieve logs from the current or most recent day

These interfaces resolve to DatabaseLogger::read_latest, which has the following TODO in it:

    pub async fn read_latest(logs_dir: ModuleLogsDir, num_lines: Option<u32>) -> String {
        // TODO: do we want to logs from across multiple files?

        let Some(num_lines) = num_lines else {
            let path = logs_dir.today();
            // look for the most recent logfile.

In standalone deployments, it's mildly inconvenient for users to locate the correct replica dir and manually view their log files. On Maincloud, older logs are entirely inaccessible. This is unfortunate.

Possible fixes include:

1. When passed a num_lines parameter greater than the number of lines in the current file, also read the previous file.
2. Accept a date as a parameter and read logs for that day.

Start at DatabaseLogger::read_latest and trace the spacetime logs command and /v1/database/:name/logs endpoint that call it. Decide whether the completed behavior should span log files, accept a date, or both, then verify that users can retrieve logs older than the current or most recent day.