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/watchlist/is_on.rs

@@ -10,3 +10,7 @@ pub async fn execute(ctx: &AppContext, query: IsOnWatchlistQuery) -> Result<bool
     let movie_id = MovieId::from_uuid(query.movie_id);
     ctx.repos.watchlist.contains(&user_id, &movie_id).await
 }
+
+#[cfg(test)]
+#[path = "tests/is_on.rs"]
+mod tests;