Logging out ends every login session (closes #256)

A login session was a cookie with nothing kept on the server, so
logging out removed it from one browser and a copy kept working for 30
days. The session manager now keeps a random value in the database,
made when it has none and replaced by each logout; a session carries
the value current at its login and is refused once it has changed. The
logout check compares no time, so the host's clock cannot block a
login, and losing the state directory ends every login session.
Logging out is now a POST with the CSRF token, sent by a form on the
generator page.

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,