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/import/tests/cleanup.rs

@@ -0,0 +1,18 @@
+use std::sync::Arc;
+
+use domain::testing::InMemoryImportSessionRepository;
+
+use crate::import::cleanup;
+use crate::test_helpers::TestContextBuilder;
+
+#[tokio::test]
+async fn returns_zero_when_nothing_expired() {
+    let sessions = InMemoryImportSessionRepository::new();
+    let ctx = TestContextBuilder::new()
+        .with_import_sessions(Arc::clone(&sessions) as _)
+        .build();
+
+    let result = cleanup::execute(&ctx).await.unwrap();
+
+    assert_eq!(result, 0);
+}