save server_name separately in the database

And use it to assert that the configured `server_name` doesn't change
after creating the database.

This prevents us from relying on the admin bot for this.

src/error.rs

@@ -75,6 +75,9 @@ pub(crate) enum ServeCommand {
 #[allow(missing_docs)]
 #[derive(Error, Debug)]
 pub(crate) enum ServerNameChanged {
+    #[error("failed to read saved server_name")]
+    ReadSavedServerName(#[source] crate::utils::error::Error),
+
     #[error("failed to check if there are any users")]
     NonZeroUsers(#[source] crate::utils::error::Error),
 
@@ -83,6 +86,9 @@ pub(crate) enum ServerNameChanged {
 
     #[error("`server_name` in the database and config file differ")]
     Renamed,
+
+    #[error("failed to save the configured server_name")]
+    SaveServerName(#[source] crate::utils::error::Error),
 }
 
 /// Observability initialization errors