add nix flake devshell and process-compose

mirrors youngstartup setup. flake provides php 8.4, composer,
nodejs, postgresql, cypress, typescript, process-compose. shellHook
seeds a per-repo postgres cluster at .postgres/. process-compose
orchestrates postgres + backend (laravel) + vite (vue spa) for
local dev. .envrc auto-loads the flake via direnv. mailpit and
gitlab-ci-local omitted - not needed for tide blogging app.

flake.nix

@@ -0,0 +1,48 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOs/nixpkgs/nixos-unstable";
+    utils.url = "github:numtide/flake-utils";
+  };
+  outputs = { self, nixpkgs, utils }:
+    utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in {
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            onefetch
+            php
+            phpPackages.composer
+            phpPackages.php-codesniffer
+            vscode-langservers-extracted
+            sqlite
+            nodejs
+            nixfmt-rfc-style
+            nixfmt-tree
+            cypress
+            yaml-language-server
+            typescript
+            postgresql
+            process-compose
+          ];
+
+          shellHook = ''
+            export PGDATA="$PWD/.postgres"
+            export PGHOST="$PGDATA"
+            export PGUSER="postgres"
+            export PGDATABASE="postgres"
+
+            if [ ! -d "$PGDATA" ]; then
+              echo "[pg] initializing cluster at $PGDATA"
+              initdb --auth=trust --username=postgres --no-locale --encoding=UTF8 >/dev/null
+              {
+                echo "listen_addresses = '127.0.0.1'"
+                echo "unix_socket_directories = '$PGDATA'"
+              } >> "$PGDATA/postgresql.conf"
+            fi
+
+            echo "[dev] run 'process-compose up' to start postgres + backend + vite"
+          '';
+        };
+      });
+}