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.

tests/Feature/V5DisabledModelIsolationTest.php

@@ -0,0 +1,22 @@
+<?php
+
+use App\Models\Environment;
+use App\Models\Project;
+use App\Models\Team;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\Support\V5TestSchema;
+
+uses(RefreshDatabase::class);
+
+it('keeps v4 project and environment checks working without the v5 schema', function () {
+    config()->set('v5.enabled', false);
+
+    $team = Team::factory()->create();
+    $project = Project::factory()->create(['team_id' => $team->id]);
+    $environment = Environment::factory()->create(['project_id' => $project->id]);
+
+    V5TestSchema::dropAllTables();
+
+    expect($project->isEmpty())->toBeTrue()
+        ->and($environment->isEmpty())->toBeTrue();
+});