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/services/search_service.rs

@@ -323,7 +323,7 @@ impl SearchUseCase for SearchService {
                 .search_folders(
                     criteria.folder_id.as_deref(),
                     criteria.name_contains.as_deref(),
-                    &user_id_str,
+                    user_id,
                     false,
                 )
                 .await?;
@@ -405,7 +405,7 @@ impl SearchUseCase for SearchService {
             .search_folders(
                 criteria.folder_id.as_deref(),
                 criteria.name_contains.as_deref(),
-                &user_id_str,
+                user_id,
                 true,
             )
             .await?;