minor: Remove unused warnings

src/db/group.rs

@@ -1,5 +1,7 @@
 #[derive(Clone, Debug)]
 pub struct Group {
+    #[expect(unused)]
     name: String,
+    #[expect(unused)]
     domain: String,
 }

src/db/memory.rs

@@ -6,6 +6,7 @@ use crate::db::{Database, DatabaseInterface, Group, User, UserRef};
 #[derive(Clone, Debug, Default)]
 pub struct MemoryDatabase {
     pub users: Vec<User>,
+    #[expect(unused)]
     pub groups: Vec<Group>,
 }