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/settings_store.py

@@ -7,7 +7,7 @@ from .migrations import DEFAULT_SETTINGS, run_migrations
 
 
 INT_KEYS = {"default_retention_value", "max_concurrent_backups"}
-BOOL_KEYS = {"setup_complete"}
+BOOL_KEYS = {"setup_complete", "keep_local_backups"}
 
 
 def parse_value(key: str, value: str):