add pdo_pgsql to backend php

Laravel connects to the production postgres database through PDO,
so the runtime interpreter must include pdo_pgsql. Without it,
tide-migrate fails on activation with 'could not find driver' and
phpfpm-tide never starts.

nix/packages/backend.nix

@@ -1,6 +1,13 @@
 { php84, lib }:
 let
-  php = php84;
+  # Laravel connects to Postgres via PDO, so the runtime PHP must
+  # ship pdo_pgsql. The default php84 set ships pdo, pdo_mysql, and
+  # pdo_sqlite but not pdo_pgsql. buildEnv gives us a php with the
+  # extra extension that flows through to passthru.php and the
+  # composer build hooks.
+  php = php84.buildEnv {
+    extensions = ({ enabled, all }: enabled ++ [ all.pdo_pgsql ]);
+  };
 in
 php.buildComposerProject (finalAttrs: {
   pname = "tide-backend";