add a read-only mode to prevent writing to the database

[MoralCode]

this can be useful for upgrades, migrations, backups .etc to avoid requiring complete downtime since limiting write access will only affect a small portion of users (admins) who likely aren't doing admin tasks that frequently anyway.

• allow for read-only mode to be toggled without restarting the API
• allow the API to proactively communicate whether it is in read only mode so the frontend can display a notice to users so they don't attempt to make any new changes