refactor(backups): simplify backups module

hosts/andromache/default.nix

@@ -68,6 +68,7 @@ in
     nixSigningKey.enable = true;
   };
 
+  restic-backup.enable = true;
   tailscale.enable = true;
 
   docker.user = config.host.username;

hosts/astyanax/default.nix

@@ -64,6 +64,7 @@ in
     nixSigningKey.enable = true;
   };
 
+  restic-backup.enable = true;
   tailscale.enable = true;
   docker.user = config.host.username;
   nfc.user = config.host.username;