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/file_retrieval_service.rs

@@ -61,7 +61,7 @@ impl FileRetrievalService {
         mime: &str,
         file_size: u64,
         accept_webp: bool,
-    ) -> Option<(Bytes, String)> {
+    ) -> Option<(Bytes, Arc<str>)> {
         if !accept_webp {
             return None;
         }
@@ -81,7 +81,7 @@ impl FileRetrievalService {
                     transcoded.len(),
                     (1.0 - transcoded.len() as f64 / content.len().max(1) as f64) * 100.0
                 );
-                Some((transcoded, webp_mime))
+                Some((transcoded, Arc::from(&*webp_mime)))
             }
             _ => None,
         }
@@ -152,7 +152,7 @@ impl FileRetrievalService {
             // Store in cache
             if let Some(cache) = &self.content_cache {
                 let etag: Arc<str> = format!("\"{}-{}\"", id, modified_at).into();
-                let ct: Arc<str> = Arc::from(&*mime_type);
+                let ct: Arc<str> = mime_type.clone();
                 cache
                     .put(id.to_string(), content_bytes.clone(), etag, ct)
                     .await;

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,