Add Nix flake for dev environment

Provides PHP with imagick, pdo_mysql, pdo_sqlite extensions, composer,
and sqlite CLI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.nix

@@ -0,0 +1,47 @@
+{
+  description = "HashRU Pics dev environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      forAllSystems = f: nixpkgs.lib.genAttrs [
+        "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin"
+      ] (system: f nixpkgs.legacyPackages.${system});
+
+      php = pkgs: pkgs.php.buildEnv {
+        extensions = { enabled, all }: enabled ++ (with all; [
+          imagick
+          pdo_mysql
+          pdo_sqlite
+        ]);
+        extraConfig = ''
+          memory_limit = 256M
+          upload_max_filesize = 50M
+          post_max_size = 50M
+        '';
+      };
+    in
+    {
+      devShells = forAllSystems (pkgs: {
+        default = pkgs.mkShell {
+          buildInputs = [
+            (php pkgs)
+            (php pkgs).packages.composer
+            pkgs.sqlite
+          ];
+
+          shellHook = ''
+            export COMPOSER_HOME="$PWD/.composer"
+
+            if [ ! -d vendor ]; then
+              echo "Running composer install..."
+              composer install
+            fi
+          '';
+        };
+      });
+    };
+}