fix: Various API model fixes

[tazlin]

In the course of expanding the SDK, certain inconsistencies were made apparent. This PR serves to correct the issues.

• fix: use SinglePeriodTxtStat for text totals stat endpoint
  • Previously, probably due to copy and pasting, the /v2/stats/text/totals endpoint published in the swagger doc that returned elements were of type SinglePeriodImgStat, which could be a source of confusion when consuming the swagger doc. /v2/stats/img/totals returns elements with different attributes, so this PR hopefully will clarify that the two endpoints return two different types of objects.
• fix: set min_length for image attr in ExtraSourceImage
  • This accurately reflects that it is a required field for creating a ExtraSourceImage
• fix: include customizer on user info endpoints
• fix: match PUT horde mode arg names to ret. payload
  • The input parameters names did not match the returned fields for the PUT action of /v2/status/modes. For clarity (and ease of object/model reuse client-side) I am changing the payload argument names to match. This is a breaking change, but as only admins (i.e., you) can use it, I think it is safe to do so.
• fix: match apidoc for ModelSingle to actual return type
  • While this endpoint should return a single value, in reality it does (and probably always has) return a list of a single element instead. Because changing the actual return value constitutes a breaking change, I recommend simply fixing the API doc for now and changing it to the (presumably) intended type when we go to /v3/.
• ci: allow SDK async tests to run
  • Forthcoming AI-Horde CI SDK-based tests will require pytest-asyncio as a reqs.dev.txt dependency.

[db0]

LGTM until now

[tazlin]

@db0 Please review these changes at your convenience. Nothing should be a breaking change but you if you have any concerns please feel free to raise them. Otherwise, please merge and deploy when you are willing. A pending SDK change would benefit in testing against production with the changes in this PR.