Add local backup retention: keep-local toggle, download, delete-local

Lets operators opt into keeping the local archive after upload (global
keep_local_backups setting), pull a remote archive back down into the
Proxmox dump directory for manual restore, and drop a locally-retained
copy without touching the remote object. Consolidates deletion so
retention, job-cascade delete, manual delete, and crash-recovery resume
all clean up local copies alongside remote ones.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/notifications.py

@@ -18,6 +18,8 @@ COLORS = {
     "backup_failed": 0xE74C3C,
     "remote_deleted": 0xF39C12,
     "recovery_action": 0x95A5A6,
+    "local_downloaded": 0x1ABC9C,
+    "local_deleted": 0x95A5A6,
 }