add health readiness API endpoint

[danangmassandy]

Fix https://github.com/unicef-drp/GeoRepo/issues/1264

Endpoint: GET /ready/

Response:

HTTP 200 OK
Allow: GET, OPTIONS
Content-Type: application/json
Vary: Accept

{
    "status": "ready",
    "checks": {
        "database": true,
        "redis": true,
        "storage": true
    },
    "storage_info": {
        "logs": {
            "path": "/home/web/logs",
            "total_gb": 434.36,
            "used_gb": 344.81,
            "free_gb": 67.42,
            "usage_percent": 79.38
        },
        "media": {
            "path": "/home/web/media",
            "total_gb": 434.36,
            "used_gb": 344.81,
            "free_gb": 67.42,
            "usage_percent": 79.38
        },
        "static": {
            "path": "/home/web/static",
            "total_gb": 434.36,
            "used_gb": 344.81,
            "free_gb": 67.42,
            "usage_percent": 79.38
        }
    }
}