feat: add backend status in the landing page GET /

[dalpasso]

Important: this PR uses the branch dynamic-monitoring-of-collections as base. Leaving as draft until this branch is merged. See #108

Report the status (online/offline) of the backends in the landing page.

A backend is considered as online if the number of collection it provides is above a given threshold (set via setting provider_online_status_threshold).

The following properties will be made available for each backend:

• status: value "online" or "offline";
• last_status_check: the most recent status check among all the collections of the provider;
• last_successful_check: the most recent successful status check among all the collections of the provider. None is all collections are offline.

If no status information is available for all the collections of the provider, then these properties are omitted.

Example of a fragment of the response of GET /. The status is missing from the provider peps cause no status info is available for it.

  "federation": {
    "wekeo_ecmwf": {
      "title": "wekeo",
      "description": "WEkEO - ECMWF data",
      "url": "https://www.wekeo.eu/",
      "status": "online",
      "last_status_check": "2026-04-23T07:24:31Z",
      "last_successful_check": "2026-04-23T07:24:31Z"
    },
    "peps": {
      "title": "peps",
      "description": "The PEPS platform, the French \"mirror site\", redistributes the products of Sentinel satellites,\nS1A, S1B, S2A and S2B, S3A and S3B from COPERNICUS, the European system for the Earth monitoring.\n",
      "url": "https://peps.cnes.fr"
    },
  }