feat: Basic domain creation API

src/db/domain.rs

@@ -0,0 +1,35 @@
+use crate::db::error::UserCreationError;
+use crate::db::{Group, User, UserRef};
+
+#[derive(Clone, Debug, Default)]
+pub struct Domain {
+    pub name: String,
+    pub users: Vec<User>,
+    #[expect(unused)]
+    pub groups: Vec<Group>,
+}
+
+impl Domain {
+    pub fn get_user(&self, req_user: &UserRef) -> Option<User> {
+        assert!(
+            req_user.domain.as_deref().unwrap_or("") == self.name,
+            "Should only call Domain::get_user on the matching domain. Asked for {:?} on domain {}",
+            req_user,
+            self.name
+        );
+
+        self.users
+            .iter()
+            .find(|u| u.username == req_user.username)
+            .cloned()
+    }
+
+    pub fn create_user(&mut self, user: User, user_ref: UserRef) -> Result<(), UserCreationError> {
+        if self.get_user(&user_ref).is_some() {
+            return Err(UserCreationError::UserAlreadyExists(user_ref));
+        }
+
+        self.users.push(user);
+        Ok(())
+    }
+}