refactor: move go-chi/captcha into Gitea (#39529)

The `gitea.com/go-chi/captcha` package only exists for Gitea,
so it moves into `modules/imagecaptcha`.

- Reloading an expired challenge shows a new image instead of a broken one
- Every answer is consumed on its first check
- OpenID registration stops after a failed captcha

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

go.mod

@@ -7,7 +7,6 @@ toolchain go1.27.1
 require (
 	connectrpc.com/connect v1.21.0
 	gitea.com/go-chi/cache v0.2.1
-	gitea.com/go-chi/captcha v0.0.0-20240315150714-fb487f629098
 	gitea.com/go-chi/session v0.0.0-20260708011333-ebced8a7a2d6
 	gitea.com/lunny/dingtalk_webhook v0.0.0-20171025031554-e3534c89ef96
 	gitea.com/lunny/levelqueue v0.4.2-0.20230414023320-3c0159fe0fe4