Implements OIDC RP-Initiated Logout (#36724)

At logout time, if the user authenticated via OIDC, we look up the
provider's `end_session_endpoint` (already discovered by Goth from the
OIDC metadata) and redirect there with `client_id` and
`post_logout_redirect_uri`.

Non-OIDC OAuth2 providers (GitHub, GitLab, etc.) are unaffected — they
fall back to local-only logout.

Fix #14270 

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Signed-off-by: Nikita Vakula <nikita.vakula@alpsalpine.com>
Co-authored-by: Nikita Vakula <nikita.vakula@alpsalpine.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/web.go

@@ -691,7 +691,7 @@ func registerWebRoutes(m *web.Router) {
 		m.Post("/recover_account", auth.ResetPasswdPost)
 		m.Get("/forgot_password", auth.ForgotPasswd)
 		m.Post("/forgot_password", auth.ForgotPasswdPost)
-		m.Post("/logout", auth.SignOut)
+		m.Get("/logout", auth.SignOut)
 		m.Get("/stopwatches", reqSignIn, user.GetStopwatches)
 		m.Get("/search_candidates", optExploreSignIn, user.SearchCandidates)
 		m.Group("/oauth2", func() {