Rename sequelize driver to db

[knolleary]

https://github.com/flowforge/flowforge-file-server/blob/0a6470e451f4aa3bed0e436c63e1ffaa4d75e5c0/README.md?plain=1#L171

We should rename the option for the database-based context backend. It should be db. Sequelize is an implementation detail that isn't meaningful to anyone configuring this.

• Update driver name in the code
• Still support 'sequelize' as a config option, map it to 'db'
• Update docs both in this repo and main user docs