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/ports/storage_ports.rs

@@ -57,10 +57,9 @@ pub trait FileReadPort: Send + Sync + 'static {
         owner_id: Uuid,
     ) -> Result<Vec<File>, DomainError> {
         let all = self.list_files(folder_id).await?;
-        let owner_str = owner_id.to_string();
         Ok(all
             .into_iter()
-            .filter(|f| f.owner_id().is_some_and(|o| o == owner_str))
+            .filter(|f| f.owner_id() == Some(owner_id))
             .collect())
     }
 
@@ -142,10 +141,9 @@ pub trait FileReadPort: Send + Sync + 'static {
     ) -> Result<Vec<File>, DomainError> {
         // Default: filter in-memory (repos should override with SQL)
         let all = self.list_files_batch(folder_id, offset, limit).await?;
-        let owner_str = owner_id.to_string();
         Ok(all
             .into_iter()
-            .filter(|f| f.owner_id().is_some_and(|o| o == owner_str))
+            .filter(|f| f.owner_id() == Some(owner_id))
             .collect())
     }