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/tests/update_settings.rs

@@ -0,0 +1,32 @@
+use std::sync::Arc;
+
+use domain::testing::InMemoryUserSettingsRepository;
+use uuid::Uuid;
+
+use crate::{
+    test_helpers::TestContextBuilder,
+    users::{get_settings, update_settings::UpdateUserSettingsCommand},
+};
+
+#[tokio::test]
+async fn updates_federate_goals() {
+    let settings_repo = InMemoryUserSettingsRepository::new();
+    let ctx = TestContextBuilder::new()
+        .with_user_settings(Arc::clone(&settings_repo) as _)
+        .build();
+
+    let uid = Uuid::nil();
+
+    crate::users::update_settings::execute(
+        &ctx,
+        UpdateUserSettingsCommand {
+            user_id: uid,
+            federate_goals: true,
+        },
+    )
+    .await
+    .unwrap();
+
+    let settings = get_settings::execute(&ctx, uid).await.unwrap();
+    assert!(settings.federate_goals());
+}