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/ports/file_ports.rs

@@ -93,7 +93,7 @@ pub enum OptimizedFileContent {
     /// Small-file content (possibly transcoded / compressed) already in RAM.
     Bytes {
         data: Bytes,
-        mime_type: String,
+        mime_type: Arc<str>,
         was_transcoded: bool,
     },
     /// Memory-mapped file (10–100 MB).