domain: introduce UserId to make uid case insensitive

Note that if there was a non-lowercase user already in the DB, it cannot
be found again. To fix this, run in the DB:

sqlite> UPDATE users SET user_id = LOWER(user_id);

server/src/domain/error.rs

@@ -3,7 +3,7 @@ use thiserror::Error;
 #[allow(clippy::enum_variant_names)]
 #[derive(Error, Debug)]
 pub enum DomainError {
-    #[error("Authentication error for `{0}`")]
+    #[error("Authentication error: `{0}`")]
     AuthenticationError(String),
     #[error("Database error: `{0}`")]
     DatabaseError(#[from] sqlx::Error),