Added customer_nr to database and invoice templates.

Used standard DATEV format(10000 – 69999)

Fixes #31

database/migrations/2025_09_17_000000_create_customers_table.php

@@ -11,6 +11,7 @@ public function up()
             $table->id();
             $table->enum('type', ['private', 'business']);
             $table->string('company_name', 100)->nullable();
+            $table->integer('customer_nr', false, true)->nullable();
             $table->string('vat_id', 50)->nullable();
             $table->string('tax_id', 50)->nullable();
             $table->string('global_id', 50)->nullable();