feat: introduce multi-tenancy by adding a structures table, structure_id to key models, and updating seeders for structure management.

database/migrations/0000_00_00_000000_create_structures_table.php

@@ -0,0 +1,31 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('structures', function (Blueprint $table) {
+            $table->id();
+            $table->string('name');
+            $table->string('slug')->unique();
+            $table->string('domain')->nullable()->unique();
+            $table->boolean('is_active')->default(true);
+            $table->timestamps();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('structures');
+    }
+};