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/search/execute.rs

@@ -7,3 +7,7 @@ use domain::{
 pub async fn execute(ctx: &AppContext, query: SearchQuery) -> Result<SearchResults, DomainError> {
     ctx.repos.search_port.search(&query).await
 }
+
+#[cfg(test)]
+#[path = "tests/execute.rs"]
+mod tests;