Add Billing/Stripe integration and Landing Page (Phase 4+5)

Phase 4 - Billing/Stripe:
- src/Billing/StripeService.php: Stripe API wrapper
  - Checkout session creation
  - Customer management
  - Billing portal sessions
  - Webhook signature verification
- src/Billing/SubscriptionManager.php: Subscription logic
  - Plan management (CRUD)
  - Trial handling
  - Feature access checks
  - Invoice storage
- src/Billing/WebhookHandler.php: Stripe webhook processing
  - checkout.session.completed
  - customer.subscription.* events
  - invoice.paid / payment_failed
- api/stripe-webhook.php: Webhook endpoint
- dashboard/billing.php: Billing dashboard
  - Current plan display with features
  - Plan comparison grid
  - Upgrade buttons with Stripe Checkout
  - Invoice history

Phase 5 - Landing Page:
- landing/index.php: Marketing homepage
  - Hero section with CTA
  - Feature grid (6 features)
  - How it works (3 steps)
  - Final CTA section
  - Responsive design
- landing/pricing.php: Pricing page
  - Dynamic plan cards from DB
  - Monthly/yearly toggle (2 months free)
  - Feature comparison
  - FAQ accordion

All features respect saas_features toggles in settings.

aurora-livecam/api/stripe-webhook.php

@@ -0,0 +1,56 @@
+<?php
+/**
+ * Stripe Webhook Endpoint
+ *
+ * URL: /api/stripe-webhook.php
+ * Konfigurieren Sie diesen Endpoint in Ihrem Stripe Dashboard
+ */
+
+// Keine Session, keine Ausgabe vor JSON
+error_reporting(0);
+ini_set('display_errors', 0);
+
+require_once dirname(__DIR__) . '/vendor/autoload.php';
+
+if (file_exists(dirname(__DIR__) . '/src/bootstrap.php')) {
+    require_once dirname(__DIR__) . '/src/bootstrap.php';
+}
+
+use AuroraLivecam\Billing\WebhookHandler;
+
+// Nur POST erlaubt
+if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
+    http_response_code(405);
+    echo json_encode(['error' => 'Method not allowed']);
+    exit;
+}
+
+// Payload lesen
+$payload = file_get_contents('php://input');
+$signature = $_SERVER['HTTP_STRIPE_SIGNATURE'] ?? '';
+
+if (empty($payload)) {
+    http_response_code(400);
+    echo json_encode(['error' => 'Empty payload']);
+    exit;
+}
+
+// Webhook verarbeiten
+try {
+    $handler = new WebhookHandler();
+    $result = $handler->handle($payload, $signature);
+
+    if ($result['success']) {
+        http_response_code(200);
+    } else {
+        http_response_code(400);
+    }
+
+    header('Content-Type: application/json');
+    echo json_encode($result);
+
+} catch (\Exception $e) {
+    error_log('Stripe Webhook Error: ' . $e->getMessage());
+    http_response_code(500);
+    echo json_encode(['error' => 'Internal server error']);
+}