feat(auth): auto-join OAuth users to root team

Add an OAuth setting that provisions newly registered users as Root
team members without creating a personal team.

database/migrations/2026_06_23_151229_add_auto_join_root_team_to_oauth_settings_table.php

@@ -0,0 +1,28 @@
+<?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::table('oauth_settings', function (Blueprint $table) {
+            $table->boolean('auto_join_root_team')->default(false);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('oauth_settings', function (Blueprint $table) {
+            $table->dropColumn('auto_join_root_team');
+        });
+    }
+};