fix(images): idempotent download (skip if file exists) + real failure reason

Retrying a template build failed at download because the qcow2 from a prior
attempt already existed and download-url refuses to overwrite. Now skip the
download when the volume already exists (volume_exists), and surface the PVE
task-log tail on download failure instead of a generic message.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/proxmox/mixins/storage.py

@@ -119,6 +119,17 @@ class StorageMixin:
                 logger.error(f"Ошибка загрузки {url} на {node}:{storage}: {e}")
                 raise
 
+        def volume_exists(self, node: str, storage: str, volid: str) -> bool:
+            """Проверить, есть ли уже том volid в хранилище (для идемпотентной загрузки)."""
+            if not self.proxmox:
+                return False
+            try:
+                items = self.proxmox.nodes(node).storage(storage).content.get()
+                return any(it.get('volid') == volid for it in (items or []))
+            except Exception as e:
+                logger.debug(f"volume_exists check failed {storage}/{volid}: {e}")
+                return False
+
         def get_download_target_storages(self, node: str, content: str) -> List[Dict]:
             """
             Получить активные хранилища ноды, поддерживающие указанный тип контента