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>,

src/application/ports/inbound.rs

@@ -1,4 +1,5 @@
 use async_trait::async_trait;
+use std::sync::Arc;
 
 use crate::application::dtos::folder_dto::{
     CreateFolderDto, FolderDto, MoveFolderDto, RenameFolderDto,
@@ -92,13 +93,15 @@ pub trait FolderUseCase: Send + Sync + 'static {
 pub trait SearchUseCase: Send + Sync + 'static {
     /// Performs a full search based on the specified criteria.
     ///
+    /// Returns `Arc<SearchResultsDto>` so the cache and the caller share
+    /// the same allocation — zero-copy on both insert and hit.
     /// `user_id` identifies the authenticated user so that SQL queries filter
     /// by owner and the result cache is isolated per tenant.
     async fn search(
         &self,
         criteria: SearchCriteriaDto,
         user_id: &str,
-    ) -> Result<SearchResultsDto, DomainError>;
+    ) -> Result<Arc<SearchResultsDto>, DomainError>;
 
     /// Returns quick suggestions for autocomplete (lightweight, fast).
     async fn suggest(

src/application/services/search_service.rs

@@ -42,9 +42,9 @@ pub struct SearchService {
     folder_repository: Arc<dyn FolderStoragePort>,
 
     /// Lock-free concurrent cache with automatic TTL and LRU eviction (moka).
-    /// Uses `future::Cache` so `.get()` / `.insert()` never block the Tokio
+    /// Values are `Arc<SearchResultsDto>` so cache insert/hit is a single
-    /// runtime (maintenance work is deferred to an internal async task).
+    /// atomic ref-count increment (~1 ns) instead of cloning thousands of Strings.
-    search_cache: moka::future::Cache<u64, SearchResultsDto>,
+    search_cache: moka::future::Cache<u64, Arc<SearchResultsDto>>,
 }
 
 // ─── Utility functions (pure, no self — computed on the server) ─────────
@@ -134,12 +134,12 @@ impl SearchService {
     }
 
     /// Attempts to retrieve results from the cache.
-    async fn get_from_cache(&self, key: u64) -> Option<SearchResultsDto> {
+    async fn get_from_cache(&self, key: u64) -> Option<Arc<SearchResultsDto>> {
         self.search_cache.get(&key).await
     }
 
     /// Stores results in the cache.
-    async fn store_in_cache(&self, key: u64, results: SearchResultsDto) {
+    async fn store_in_cache(&self, key: u64, results: Arc<SearchResultsDto>) {
         self.search_cache.insert(key, results).await;
     }
 
@@ -272,7 +272,7 @@ impl SearchUseCase for SearchService {
      * - Human-readable size formatting
      * - Pagination
      */
-    async fn search(&self, criteria: SearchCriteriaDto, user_id: &str) -> Result<SearchResultsDto> {
+    async fn search(&self, criteria: SearchCriteriaDto, user_id: &str) -> Result<Arc<SearchResultsDto>> {
         let start = Instant::now();
 
         // Try to get from cache
@@ -359,7 +359,7 @@ impl SearchUseCase for SearchService {
 
             let elapsed_ms = start.elapsed().as_millis() as u64;
 
-            let search_results = SearchResultsDto::new(
+            let search_results = Arc::new(SearchResultsDto::new(
                 paginated_files,
                 paginated_folders,
                 criteria.limit,
@@ -367,9 +367,9 @@ impl SearchUseCase for SearchService {
                 Some(total_count),
                 elapsed_ms,
                 criteria.sort_by.clone(),
-            );
+            ));
 
-            self.store_in_cache(cache_key, search_results.clone()).await;
+            self.store_in_cache(cache_key, Arc::clone(&search_results)).await;
             return Ok(search_results);
         }
 
@@ -443,7 +443,7 @@ impl SearchUseCase for SearchService {
 
         let elapsed_ms = start.elapsed().as_millis() as u64;
 
-        let search_results = SearchResultsDto::new(
+        let search_results = Arc::new(SearchResultsDto::new(
             paginated_files,
             paginated_folders,
             criteria.limit,
@@ -451,10 +451,10 @@ impl SearchUseCase for SearchService {
             Some(total_count),
             elapsed_ms,
             criteria.sort_by.clone(),
-        );
+        ));
 
-        // Store in cache
+        // Store in cache — Arc::clone is ~1 ns (atomic increment)
-        self.store_in_cache(cache_key, search_results.clone()).await;
+        self.store_in_cache(cache_key, Arc::clone(&search_results)).await;
 
         Ok(search_results)
     }
@@ -490,8 +490,8 @@ impl SearchService {
                 &self,
                 _criteria: SearchCriteriaDto,
                 _user_id: &str,
-            ) -> Result<SearchResultsDto> {
+            ) -> Result<Arc<SearchResultsDto>> {
-                Ok(SearchResultsDto::empty())
+                Ok(Arc::new(SearchResultsDto::empty()))
             }
 
             async fn suggest(

src/common/stubs.rs

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

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);