Added customer_nr to database and invoice templates.

Used standard DATEV format(10000 – 69999)

Fixes #31

database/seeders/TooloopSeeder.php

@@ -2,6 +2,7 @@
 
 namespace Database\Seeders;
 
+use App\Http\Controllers\CustomerController;
 use Illuminate\Database\Seeder;
 use App\Models\User;
 use App\Models\Customer;
@@ -81,6 +82,7 @@ public function run(): void
             $customer = Customer::firstOrCreate(
                 ['company_name' => $row['Kunde']],
                 [
+                    'customer_nr' => CustomerController::generateCustomerNumber(),
                     'type' => 'business',
                     'vat_id' => '',
                     'tax_id' => '',