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/application/dtos/search_dto.rs

@@ -156,7 +156,7 @@ pub struct SearchFolderResultDto {
  * both files and folders that match the search criteria, along with pagination
  * information and server-computed metadata.
  */
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Serialize, Deserialize)]
 pub struct SearchResultsDto {
     /// Files matching the search criteria (enriched with metadata)
     pub files: Vec<SearchFileResultDto>,