feat(v5): gate V5 to development environments only

Introduce V5Feature and config so V5 routes, jobs, commands, morph maps,
and model queries run only when enabled. Move V5 migrations to
migrations-v5 (loaded only when enabled), remove Flux from production
Docker/install paths, and add isolation tests.

config/v5.php

@@ -0,0 +1,7 @@
+<?php
+
+use App\Support\V5\V5Feature;
+
+return [
+    'enabled' => V5Feature::enabledForEnvironment((string) env('APP_ENV', 'production')),
+];