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/common/stubs.rs

@@ -505,7 +505,7 @@ impl FileRetrievalUseCase for StubFileRetrievalUseCase {
             FileDto::default(),
             OptimizedFileContent::Bytes {
                 data: Bytes::new(),
-                mime_type: String::new(),
+                mime_type: Arc::from(""),
                 was_transcoded: false,
             },
         ))