MY-6: Add MySQLTuner diagnostic script for workload-specific tuning

[szachovy]

Problem

MY-1 through MY-5 cover the most impactful MySQL defaults, but workload-specific tuning requires profiling a live cluster under representative load. There is no tooling for this today.

Fix

Create scripts/tuning.sh:

#!/bin/bash
# Run against a live MySQL node for tuning recommendations
NODE="${1:-node-2}"
ssh superset@$NODE "docker exec mysql bash -c '
    curl -sL https://raw.githubusercontent.com/major/MySQLTuner-perl/master/mysqltuner.pl -o /tmp/mysqltuner.pl
    perl /tmp/mysqltuner.pl --login-path=root
'"

Run after the cluster has been serving representative Superset queries for at least a few hours. This is a diagnostic tool — do not add it to CI.

Acceptance Criteria

• scripts/tuning.sh created that SSHs into a MySQL node and runs MySQLTuner against the live cluster
• Script accepts node name as argument (default: node-2)
• Script documented in README or docs/ with instructions to run after representative load