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/auth/tests/register_and_login.rs

@@ -0,0 +1,22 @@
+use crate::auth::commands::RegisterAndLoginCommand;
+use crate::auth::register_and_login;
+use crate::test_helpers::TestContextBuilder;
+
+#[tokio::test]
+async fn registers_and_returns_token() {
+    let ctx = TestContextBuilder::new().build();
+
+    let result = register_and_login::execute(
+        &ctx,
+        RegisterAndLoginCommand {
+            email: "new@example.com".into(),
+            username: "newuser".into(),
+            password: "password123".into(),
+        },
+    )
+    .await
+    .unwrap();
+
+    assert!(!result.token.is_empty());
+    assert_eq!(result.email, "new@example.com");
+}