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/tests/test_backup_service.py

@@ -36,7 +36,6 @@ def _job():
             "node": "pve",
             "enabled": True,
             "cron_schedule": "0 2 * * *",
-            "proxmox_storage": "backup-store",
             "backup_mode": "snapshot",
             "compression": "zstd",
             "retention_type": "latest",