feature: the status page shows the Router Manager's version.

README.md

@@ -526,10 +526,12 @@ curl http://10.0.0.4:17081/
 | `/users` | The users of OpenVPN and road-warrior access; needs a token |
 | `/groups` | The groups of users; needs a token |
 
-The status holds the state of `vpn-router-setup` and `slapd`, of `strongswan` when site-to-site or
+The status holds the state of `vpn-router-setup`, `slapd` and each PostgreSQL cluster unit
+(`postgresql@<version>-<cluster>`, or `postgresql` as `no cluster` without one), of `strongswan` when site-to-site or
 road-warrior access is configured, of `wg-quick@wg0` when WireGuard is, and of
 `openvpn-server@vpn-router` when OpenVPN is; the number of connected
-OpenVPN clients, from the server's status file, which OpenVPN rewrites every minute; the package version; whether
+OpenVPN clients, from the server's status file, which OpenVPN rewrites every minute; the package version and,
+as `manager_version`, the installed version of `vpn-router-manager` (`null`, shown as not installed, without it); whether
 Docker is installed, and when it is, the state of `docker.service` and of `vpn-router-docker`
 (the Docker rules), which do not change `status`; whether the
 site-to-site tunnel is established; the number of connected road-warrior (IPsec P2S) clients; and