feat: Start dummy in-memory database

src/db/error.rs

@@ -0,0 +1,14 @@
+use std::fmt;
+
+use crate::db::UserRef;
+
+#[derive(Debug)]
+pub struct UserAlreadyExists(pub UserRef);
+
+impl fmt::Display for UserAlreadyExists {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "User already exists: {}", self.0)
+    }
+}
+
+impl std::error::Error for UserAlreadyExists {}