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/get_users.rs

@@ -0,0 +1,13 @@
+use crate::test_helpers::TestContextBuilder;
+use crate::users::get_users;
+use crate::users::queries::GetUsersQuery;
+
+#[tokio::test]
+async fn returns_empty_when_no_users() {
+    let ctx = TestContextBuilder::new().build();
+
+    let result = get_users::execute(&ctx, GetUsersQuery).await.unwrap();
+
+    assert!(result.users.is_empty());
+    assert!(result.remote_actors.is_empty());
+}