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.
• successful_collections: number of online collections in the provider;
• failed_collections: number of offline collections in the provider;
• total_collections: total number of collections in the provider (online, offline, no status info available).

Note that it's possible that no status information is available for some collection. In such case case successful_collections + failed_collections < total_collections.

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": {
      "description": "WEkEO - ECMWF data",
      "title": "wekeo",
      "status": "online",
      "last_status_check": "2026-05-06T17:46:33Z",
      "successful_collections": 34,
      "failed_collections": 1,
      "total_collections": 37
    }
  }