treewide: ensure services are started after secrets setup

Now all services that access secrets only run after the secrets setup
has finished.

Previously, we assumed that the systemd `after` dependency is
transitive, i.e. that adding an `after = [ "bitcoind.service" ]`
to a service implicitly pulled in the `after` dependency to
`nix-bitcoin-secrets.target` (which is defined for `bitcoind`).
This is not the case. Services could start before secrets setup
had finished, leading to service failure.

modules/fulcrum.nix

@@ -112,7 +112,7 @@ in {
     systemd.services.fulcrum = {
       wantedBy = [ "multi-user.target" ];
       requires = [ "bitcoind.service" ];
-      after = [ "bitcoind.service" ];
+      after = [ "bitcoind.service" "nix-bitcoin-secrets.target" ];
       preStart = ''
         {
           cat ${configFile}