perf: use Arc<str> for repetitive DTO fields to eliminate clone allocations

Replace String with Arc<str> for fields that contain repeated static
values (mime_type, icon_class, icon_special_class, category) in
FileDto, FolderDto, and OptimizedFileContent.

These fields are computed from ~40 static lookup tables and cloned
on every request. With Arc<str>, clone becomes O(1) atomic increment
instead of O(n) heap allocation — saving thousands of allocations/s
under load.

Fields kept as String: id, name, path, folder_id, owner_id
(unique per item, rarely cloned).

Zero API impact — serde serializes Arc<str> identically to String.

https://claude.ai/code/session_01EbAFEfyJNLRmJHmmYDX3Tt

src/application/services/search_service.rs

@@ -158,7 +158,7 @@ impl SearchService {
             name: file.name.clone(),
             path: file.path.clone(),
             size: file.size,
-            mime_type: file.mime_type.clone(),
+            mime_type: file.mime_type.to_string(),
             folder_id: file.folder_id.clone(),
             created_at: file.created_at,
             modified_at: file.modified_at,