It is good practice to create a dedicated read-only user to browse the
database safely. But since the app itself creates its tables, the
postgres user is the owner and a manual `GRANT` is required everytime a
new table is added.

This PR makes it possible to specify an arbitrary username, that will be
granted read-only access to all tables in the `public` schema.

NB: The assumption here is that eclair is the only app using the
eclair database (in the `CREATE DATABASE eclair` sense), which I believe
is reasonable.

This only affects newly created table, there is no migration.

Users that are already using postgres will keep the previous column
type, it doesn't change anything for them.

We use a timeout, because due to concurrency we may not be able to
obtain a lock immediately.

The timeout has been set to its original value of 5s and made
configurable.

From Hikari doc:
> 🔤connectionInitSql
This property sets a SQL statement that will be executed after every new
connection creation before adding it to the pool. If this SQL is not
valid or throws an exception, it will be treated as a connection failure
and the standard retry logic will be followed. Default: none