ramus, tangra, flake: apply php backport overlay

Covers every PHP consumer: think/think-backend (php83) on ramus,
wordpress pools (pkgs.php) on tangra, and the perSystem think packages.

machines/tangra/configuration.nix

@@ -1,3 +1,4 @@
+{ self, ... }:
 {
   system.stateVersion = "25.11";
   nixpkgs.hostPlatform = {
@@ -29,4 +30,7 @@
   security.acme.acceptTerms = true;
 
   imports = [ ];
+
+  # TEMPORARY: wordpress php-fpm pools use pkgs.php; see overlays/nixpkgs-pr-576853-php.nix
+  nixpkgs.overlays = [ self.overlays.phpPr576853Overlay ];
 }