feat: add health endpoint

[tigattack]

Checks DB connection & lock status.

I'm largely unfamiliar with TS (and Mongo for that matter lol), but this change has achieved my goal and seems to function well. Hopefully it's suitable. Let me know what you think.

Responses

Connected and unlocked:

{
    "status": "ok",
    "db_health": {
        "status": "UP",
        "details": {
            "version": "6.0.19",
            "uptime": 6843,
            "writable": true
        }
    }
}

Connected and locked:

{
    "status": "warning",
    "db_health": {
        "status": "READONLY",
        "details": {
            "version": "6.0.19",
            "uptime": 7244,
            "writable": false
        },
        "message": "Database is read-only (fsyncLock enabled)"
    }
}

Disconnected:

{
    "status": "error",
    "db_health": {
        "status": "DOWN",
        "message": "Error querying database health"
    }
}

[tigattack]

On second thoughts, perhaps READONLY should be WARNING... Open to thoughts.