`fly postgres backup config` resets after image update

The issue names the fly postgres backup config, fly image update, and f config show commands but no source file or test. Reproduce the image update on a PostgreSQL app, compare backup settings before and after, then trace the command entry points and update flow. Done means backup configuration remains unchanged after the image update, with a regression test covering that sequence.

Describe the bug

after I upgrade postgres image with fly image update, backup configs are reset to the default value

• Operating system
• fly version

❯ f version
fly v0.3.110 darwin/arm64 Commit: 31e7022b2ed3b929fc05826dde1796681d9229de BuildDate: 2025-04-24T18:18:02Z

** Paste your fly.toml

let me paste json instead since we don't have toml file for our app

❯ f config show -a production-nexus-db

{
    "app": "production-myapp-db",
    "primary_region": "fra",
    "env": {
        "FLY_RESTORED_FROM": "vol_4y7w3lnj7n7ejqj4",
        "PRIMARY_REGION": "fra"
    },
    "mounts": [
        {
            "source": "pg_data",
            "destination": "/data"
        }
    ],
    "services": [
        {
            "protocol": "tcp",
            "internal_port": 5432,
            "auto_start_machines": false,
            "ports": [
                {
                    "port": 5432,
                    "handlers": [
                        "pg_tls"
                    ]
                }
            ],
            "concurrency": {
                "type": "connections",
                "hard_limit": 1000,
                "soft_limit": 1000
            }
        },
        {
            "protocol": "tcp",
            "internal_port": 5433,
            "auto_start_machines": false,
            "ports": [
                {
                    "port": 5433,
                    "handlers": [
                        "pg_tls"
                    ]
                }
            ],
            "concurrency": {
                "type": "connections",
                "hard_limit": 1000,
                "soft_limit": 1000
            }
        }
    ],
    "checks": {
        "pg": {
            "port": 5500,
            "type": "http",
            "interval": "15s",
            "timeout": "10s",
            "path": "/flycheck/pg"
        },
        "role": {
            "port": 5500,
            "type": "http",
            "interval": "15s",
            "timeout": "10s",
            "path": "/flycheck/role"
        },
        "vm": {
            "port": 5500,
            "type": "http",
            "interval": "15s",
            "timeout": "10s",
            "path": "/flycheck/vm"
        }
    },
    "metrics": [
        {
            "port": 9187,
            "path": "/metrics"
        }
    ]
}