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/common/stubs.rs

@@ -246,7 +246,7 @@ impl FolderRepository for StubFolderStoragePort {
     async fn list_folders_by_owner(
         &self,
         _parent_id: Option<&str>,
-        _owner_id: &str,
+        _owner_id: Uuid,
     ) -> Result<Vec<Folder>, DomainError> {
         Ok(Vec::new())
     }
@@ -264,7 +264,7 @@ impl FolderRepository for StubFolderStoragePort {
     async fn list_folders_by_owner_paginated(
         &self,
         _parent_id: Option<&str>,
-        _owner_id: &str,
+        _owner_id: Uuid,
         _offset: usize,
         _limit: usize,
         _include_total: bool,
@@ -314,7 +314,7 @@ impl FolderRepository for StubFolderStoragePort {
 
     async fn create_home_folder(
         &self,
-        _user_id: &str,
+        _user_id: Uuid,
         _name: String,
     ) -> Result<Folder, DomainError> {
         Ok(Folder::default())