API for per-day stats

[tazz4843]

I'm looking to bring my analytics data into Grafana since I already have everything else there. While trying to do so, I noticed there's no real way to transform the current data on GET /stats/hits into a total sum of hits per day, given the limited number of ways I have to process the data (JSONPath and JSONata, neither of which support the transformation required to get a sum per day). I can't make multiple API requests either, it all has to be done in one request.

About the only workaround I can see is a new API endpoint that returns data something like this, combined across all routes as a total sum.

{
  "total": 1000,
  "per_day": [
    {
      "date": "2023-09-05",
      "pageviews": 18,
      "visitors": 8
    },
    {
      "date": "2023-09-06",
      "pageviews": 10,
      "visitors": 3
    },
    {
      "date": "2023-09-07",
      "pageviews": 8,
      "visitors": 1
    }
  ]
}

Adding this to the existing GET /stats/total would also be doable for my use case.

[arp242]

Somewhat related: #662 (comment)

Adding a group parameter to /stats/total would probably also make sense here.