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/api/proxmox/__init__.py

@@ -8,6 +8,7 @@ from .backup import router as backup_router
 from .cluster import router as cluster_router
 from .networks import router as networks_router
 from .lxc import router as lxc_router
+from .images import router as images_router
 from ._helpers import get_next_vmid, save_vm_instance, get_vm_instance
 
 router = APIRouter()
@@ -20,3 +21,4 @@ router.include_router(backup_router)
 router.include_router(cluster_router)
 router.include_router(networks_router)
 router.include_router(lxc_router)
+router.include_router(images_router)