Rename gem to SQLGenius

[antarr]

Summary

• Rename the gem and public brand from mysql_genius / MysqlGenius to sql_genius / SqlGenius.
• Update Rails engine namespace, mount path, generator, require paths, docs, license branding, and RubyGems publish workflow.
• Keep database-specific MySQL/MariaDB wording where it describes actual dialect behavior.

Testing

• bundle exec rspec
• gem build sql_genius.gemspec