Implement new stats endpoint

[bkis]

• path: /api/platform/stats
• returns extended stats when request is made by admin (A) or user (U) as opposed to visitor (V)
• response model in new module tekst/models/stats.py
• things that could be in scope for the endpoint:
  • number of registered users (V)
  • number of users with an active session (A)
  • number of users that logged in within the last week (needs last_login field on user doc, exclude from read model!) (U)
  • number of users that logged in within the last month (see above) (U)
  • number of users that logged in within the last year (see above) (U)
  • current number of outdated user messages (A)
  • current number of outdated access tokens (A)
  • total number of requested PW resets (happens on auth hooks; needs a stats collection for counting or a "stats" object in the state collection – which should be renamed to platform... and could the IDs just be "state" and "stats"??) (A)
  • total number of logins (A)
  • total number of quick searches (V)
  • total number of advanced searches (V)
  • total number of sent emails (A)
  • list of texts – with:
    • number of resources
    • ...
  • ...

The data should be served something like this:

{
  "users": {
    "registered": 40,
    "activeSession": 2,
    "loggedInPastWeek": 5,
    "loggedInPastMonth": 25,
    "loggedInPastYear": 39
  },
  "texts": [
    {
      "title": "Foo",
      "resources": 18
    }
  ],
  "search": {
    "countQuick": 321,
    "coundAdvanced": 210
  },
  "system": {
    "pwResets": 12,
    "oodTokens": 2,
    "oodUserMsgs": 13
  }
}