feat(user): Personal access tokens can be regenerated (#38907)

Lets users regenerate a personal access token's value in place, keeping
its name and scopes, instead of deleting and recreating it. Useful when
a token was shared with a third party (e.g. an AI agent) and needs to
be invalidated immediately without redoing scope selection.

Follows the same pattern already used for OAuth2 application client
secrets (`GenerateClientSecret`/`RegenerateSecret`).

**Testing**: added a model unit test and a web integration test;
manually
verified in the running dev server that the old token stops
authenticating
and the new one works immediately after regenerating.

<img width="1040" height="245" alt="image"
src="https://github.com/user-attachments/assets/4de0d8b4-1fc4-49cf-a859-95e24d0b2c0a"
/>

Fixes #38683.

---------

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

routers/web/web.go

@@ -695,6 +695,7 @@ func registerWebRoutes(m *web.Router, webAuth *AuthMiddleware) {
 			m.Combo("").Get(user_setting.Applications).
 				Post(web.Bind[*forms.NewAccessTokenForm](), user_setting.ApplicationsPost)
 			m.Post("/delete", user_setting.DeleteApplication)
+			m.Post("/regenerate", user_setting.RegenerateAccessToken)
 		})
 
 		m.Combo("/keys").Get(user_setting.Keys).