Add PayPal, Stripe, CSV-Export; fix .env.production issue

- PayPal: Checkout-Integration via srmklive/paypal, Weiterleitung zu PayPal,
  Bestellung wird nach Capture automatisch auf payment_received gesetzt
- Stripe: Payment Elements im Checkout, Intent-API, Zahlung direkt bestätigt
- CSV-Export: Admin → Bestellungen → CSV exportieren (UTF-8 BOM für Excel)
- PayPal/Stripe nur sichtbar wenn Keys in .env gesetzt
- .env.example um PAYPAL_* und STRIPE_* Keys erweitert

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

database/migrations/2024_01_01_000022_add_payment_fields_to_orders.php

@@ -0,0 +1,25 @@
+<?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('orders', function (Blueprint $table) {
+            $table->string('payment_id')->nullable()->after('payment_method');
+        });
+
+        // payment_method enum um paypal und stripe erweitern
+        DB::statement("ALTER TABLE orders MODIFY COLUMN payment_method ENUM('prepayment','invoice','paypal','stripe') NOT NULL DEFAULT 'prepayment'");
+    }
+
+    public function down(): void
+    {
+        Schema::table('orders', function (Blueprint $table) {
+            $table->dropColumn('payment_id');
+        });
+        DB::statement("ALTER TABLE orders MODIFY COLUMN payment_method ENUM('prepayment','invoice') NOT NULL DEFAULT 'prepayment'");
+    }
+};