add nix development environment

Provide a direnv-loaded Laravel and future Vue toolchain with isolated worktree ports and a process-compose stack. Use PostgreSQL for local runtime services while retaining in-memory SQLite for PHPUnit.

backend/composer.json

@@ -81,7 +81,6 @@
         ],
         "post-create-project-cmd": [
             "@php artisan key:generate --ansi",
-            "@php -r \"file_exists('database/database.sqlite') || touch('database/database.sqlite');\"",
             "@php artisan migrate --graceful --ansi"
         ],
         "pre-package-uninstall": [