feat(images): backend catalog + mirror model + download API/worker

Phase 1 of VM image mirror feature:
- StorageMixin.download_url (PVE download-url/aria2) + get_download_target_storages
- builtin curated cloud-image catalog (Ubuntu/Debian/Rocky/Alma, amd64+arm64)
- ImageMirror model for admin custom mirrors
- /proxmox/api/images: catalog, lxc-templates proxy, storages, download
- _do_image_download_sync worker with log-tailing progress (DeployTask)

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

backend/app/models/tasks.py

@@ -143,6 +143,8 @@ class DeployTask(Base):
             'reinstall': f"Переустановка {self.name}",
             'clone': f"Клонирование {self.name}",
             'change_password': f"Смена пароля: {self.name}",
+            'image_download': f"Загрузка образа: {self.name}",
+            'image_template': f"Шаблон из образа: {self.name}",
         }
         return {
             'id': self.id,