Use transactions for endpoints that have multiple database operations that may fail

[rjwebb]

The existing codebase doesn't use database transactions for anything, so there are certain operations (such as creating a new user) that may fail, leaving the database in an invalid state.

For example, the "create user" function in https://github.com/canvasxyz/metropolis/blob/main/server/src/auth/create-user.ts#L13 consists of a number of steps that all need to take place successfully:

• check if a user exists with the same email - this is just a select query
• insert the new user entry - this may fail if there has been a race condition and a new user has been created since the previous step took place
• insert a hashed password into the database - if this step fails, we end up with a user entry that has no password, so it will be impossible to create a new user with the same email address
• create a session cookie
• add the session cookie to the response

We should just wrap the entire sequence of steps in a transaction, so that if something fails the pending changes to the database can be discarded.