perf: Arc-wrap SearchResultsDto for zero-copy cache reads

- Change SearchUseCase::search() return type to Arc<SearchResultsDto>
- Replace Cache<u64, SearchResultsDto> with Cache<u64, Arc<SearchResultsDto>>
- Eliminate .clone() on full result set; use Arc::clone() (ptr bump)
- Remove Clone derive from SearchResultsDto (no longer needed)
- Update handlers to deref Arc for JSON serialization

src/common/stubs.rs

@@ -597,8 +597,8 @@ impl SearchUseCase for StubSearchUseCase {
         &self,
         _criteria: SearchCriteriaDto,
         _user_id: &str,
-    ) -> Result<SearchResultsDto, DomainError> {
-        Ok(SearchResultsDto::empty())
+    ) -> Result<Arc<SearchResultsDto>, DomainError> {
+        Ok(Arc::new(SearchResultsDto::empty()))
     }
 
     async fn suggest(