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/infrastructure/services/path_resolver_service.rs

@@ -149,9 +149,9 @@ impl PathResolverService {
                 created_at: created_at as u64,
                 modified_at: modified_at as u64,
                 is_root: false,
-                icon_class: "fas fa-folder".to_string(),
-                icon_special_class: "folder-icon".to_string(),
-                category: "Folder".to_string(),
+                icon_class: Arc::from("fas fa-folder"),
+                icon_special_class: Arc::from("folder-icon"),
+                category: Arc::from("Folder"),
             })),
             _ => {
                 let mime = mime_type.unwrap_or_else(|| "application/octet-stream".to_string());
@@ -161,13 +161,13 @@ impl PathResolverService {
                     name: name.clone(),
                     path: res_path,
                     size: sz,
-                    mime_type: mime.clone(),
+                    mime_type: Arc::from(&*mime),
                     folder_id,
                     created_at: created_at as u64,
                     modified_at: modified_at as u64,
-                    icon_class: icon_class_for(&name, &mime).to_string(),
-                    icon_special_class: icon_special_class_for(&name, &mime).to_string(),
-                    category: category_for(&name, &mime).to_string(),
+                    icon_class: Arc::from(icon_class_for(&name, &mime)),
+                    icon_special_class: Arc::from(icon_special_class_for(&name, &mime)),
+                    category: Arc::from(category_for(&name, &mime)),
                     size_formatted: format_file_size(sz),
                     owner_id: user_id,
                 }))