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.

.envrc

@@ -0,0 +1,11 @@
+# Load the flake environment
+use flake
+
+onefetch
+
+# Use PHP and Node layouts
+layout php
+layout node
+
+# Reload when backend dependencies change
+watch_file backend/composer.json