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/interfaces/api/handlers/search_handler.rs

@@ -67,7 +67,7 @@ impl SearchHandler {
                     results.files.len(),
                     results.folders.len()
                 );
-                (StatusCode::OK, Json(results)).into_response()
+                (StatusCode::OK, Json(&*results)).into_response()
             }
             Err(err) => {
                 error!("Search error: {}", err);
@@ -108,7 +108,7 @@ impl SearchHandler {
                     results.files.len(),
                     results.folders.len()
                 );
-                (StatusCode::OK, Json(results)).into_response()
+                (StatusCode::OK, Json(&*results)).into_response()
             }
             Err(err) => {
                 error!("Search error: {}", err);