Make proxmox_storage a global setting instead of a per-job snapshot

Backup jobs stored their own proxmox_storage column, copied from
settings at creation time and never resynced. process_backup preferred
that frozen job value, so changing Settings > Proxmox storage had no
effect on existing jobs. Drop the per-job column and always use the
current global setting when starting a backup.

Also copies AGENTS.md to CLAUDE.md.

backend/app/backup_service.py

@@ -245,7 +245,7 @@ async def process_backup(backup_id: str) -> None:
                 commands.start_proxmox_backup,
                 node=backup["node"],
                 vmid=int(backup["guest_vmid"]),
-                storage=(job or {}).get("proxmox_storage") or settings["proxmox_storage"],
+                storage=settings["proxmox_storage"],
                 mode=(job or {}).get("backup_mode") or settings["default_backup_mode"],
                 compression=(job or {}).get("compression") or settings["default_compression"],
             )