fix: cascade delete namespaces when workspace is removed

Change namespaces.workspace_id FK from nullOnDelete to cascadeOnDelete
so that namespaces are properly cleaned up when their parent workspace
is deleted, instead of being orphaned with a null workspace_id.

Fixes #10

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Migrations/2026_03_24_000000_cascade_delete_namespaces_on_workspace_delete.php

@@ -0,0 +1,40 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Change namespaces.workspace_id FK from nullOnDelete to cascadeOnDelete.
+     *
+     * Fixes #10 — namespaces were orphaned with null workspace_id when
+     * the parent workspace was deleted.
+     */
+    public function up(): void
+    {
+        Schema::table('namespaces', function (Blueprint $table) {
+            $table->dropForeign(['workspace_id']);
+
+            $table->foreign('workspace_id')
+                ->references('id')
+                ->on('workspaces')
+                ->cascadeOnDelete();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('namespaces', function (Blueprint $table) {
+            $table->dropForeign(['workspace_id']);
+
+            $table->foreign('workspace_id')
+                ->references('id')
+                ->on('workspaces')
+                ->nullOnDelete();
+        });
+    }
+};