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/handlers/auth.go

@@ -89,10 +89,18 @@ func (s *Handlers) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, "/", http.StatusSeeOther)
 }
 
-// HandleLogout clears the session and redirects to login.
+// HandleLogout logs out, which ends every login session (see
+// session.Manager.Logout), and redirects to login.
 func (s *Handlers) HandleLogout() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		s.sessMgr.ClearSession(w)
+		err := s.sessMgr.Logout(w, r)
+		if err != nil {
+			s.log.Error("failed to log out", "error", err)
+			http.Error(w, "Failed to log out", http.StatusInternalServerError)
+
+			return
+		}
+
 		http.Redirect(w, r, "/", http.StatusSeeOther)
 	}
 }