Log out with a form and count logouts instead of timing them (part of #256)

Logging out is now a POST in the CSRF-protected route group, sent by a
form on the generator page, so a link or image on another page of the
same site cannot end every login session. The session manager keeps a
count of logouts instead of the time of the latest one: each session
carries the count at its login and is refused once it has changed. No
clock is compared, so a login after a logout always works.

Model: opus-5-5

internal/server/routes.go

@@ -69,10 +69,9 @@ func (s *Server) SetupRoutes() {
 		r.With(s.mw.RateLimit(LoginAttemptsPerMinute, time.Minute, nil)).
 			Post("/", s.h.HandleRoot())
 		r.Post("/generate", s.h.HandleGenerateURL())
+		r.Post("/logout", s.h.HandleLogout())
 	})
 
-	s.router.Get("/logout", s.h.HandleLogout())
-
 	// Image routes, the only ones that send CORS headers, as pages on other
 	// sites read them. They are a subrouter rather than a group: a group's
 	// middleware runs only for a request that matches one of its routes,