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/create_session.rs

@@ -0,0 +1,22 @@
+use uuid::Uuid;
+
+use crate::import::{commands::CreateImportSessionCommand, create_session};
+use crate::test_helpers::TestContextBuilder;
+
+#[tokio::test]
+async fn creates_session_with_parsed_file() {
+    let ctx = TestContextBuilder::new().build();
+
+    let result = create_session::execute(
+        &ctx,
+        CreateImportSessionCommand {
+            user_id: Uuid::new_v4(),
+            bytes: b"col1\nval1".to_vec(),
+            format: domain::models::FileFormat::Csv,
+        },
+    )
+    .await
+    .unwrap();
+
+    assert!(!result.columns.is_empty());
+}