fix(lifecycle): pass blob content_hash to file lifecycle hooks, not etag

The five FileLifecycleHook call sites handed dto.etag (format
"{blob_hash[..16]}-{modified_at}") to the blob_hash parameter. Blob
lookups keyed by that value can never resolve, so background thumbnail
generation and audio metadata extraction silently failed on every
upload/copy/update, pushing all thumbnail work onto the request path.

Pass dto.content_hash — the raw full BLAKE3 hash already carried by
FileDto — instead.

https://claude.ai/code/session_01QxwJDHqQhbMkHK333QtMme

src/application/services/file_management_service.rs

@@ -151,7 +151,7 @@ impl FileManagementService {
 
         let dto = FileDto::from(copied_file);
         if let Some(hook) = &self.file_lifecycle_hook {
-            hook.on_file_copied(&dto.id, &dto.etag, &dto.mime_type, file_id);
+            hook.on_file_copied(&dto.id, &dto.content_hash, &dto.mime_type, file_id);
         }
         Ok(dto)
     }