perf: complete storage-layer UUID migration — eliminate UUID↔String round-trips

- Domain: Folder.owner_id + File.owner_id → Option<Uuid> (was Option<String>)
- Traits: folder_repository signatures accept Uuid directly (was &str)
- Infra: remove all fi.user_id::text / fo.user_id::text SQL casts in repos
- Infra: remove $N::uuid bind casts (columns are native UUID now)
- Services: eliminate to_string() bridge variables in folder_service,
  search_service, trash_service
- DTOs: boundary conversion to String only at API edge (file_dto, folder_dto)
- Ports: default impls compare Option<Uuid> directly
- Stubs: updated signatures to match trait

13 files changed across domain/application/infrastructure layers.
Compiles clean (cargo check passes).

src/application/dtos/file_dto.rs

@@ -83,7 +83,7 @@ impl From<File> for FileDto {
             icon_special_class,
             category,
             size_formatted,
-            owner_id: parts.owner_id,
+            owner_id: parts.owner_id.map(|u| u.to_string()),
             sort_date: None,
         }
     }