add 400+ unit tests for domain and application layers

Extract ReviewLogger trait to decouple import/integrations
from diary::log_review (cross-module coupling smell).

Add in-memory fakes for all repository ports, enabling
isolated testing of every use case module without a database.

Coverage: domain+application 22% → 80%, 427 tests.

crates/application/src/users/update_settings.rs

@@ -13,3 +13,7 @@ pub async fn execute(ctx: &AppContext, cmd: UpdateUserSettingsCommand) -> Result
     settings.set_federate_goals(cmd.federate_goals);
     ctx.repos.user_settings.save(&settings).await
 }
+
+#[cfg(test)]
+#[path = "tests/update_settings.rs"]
+mod tests;