feat(treasury): allow null unit_id for franchisor network accounts

Franqueador (rede/conta-mãe) cadastra bancos sem unidade; franchisee
continua usando o unit_id da unidade ativa.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

database/migrations/2026_06_18_000001_make_unit_id_nullable_on_treasury_accounts_table.php

@@ -0,0 +1,24 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('treasury_accounts', function (Blueprint $table) {
+            // Franqueador (rede/conta-mãe) cadastra bancos sem unidade (unit_id = null).
+            // Franchisee continua usando o unit_id da unidade ativa.
+            $table->foreignId('unit_id')->nullable()->change();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('treasury_accounts', function (Blueprint $table) {
+            $table->foreignId('unit_id')->nullable(false)->change();
+        });
+    }
+};