Billing Phase 2: Stripe Checkout, Kundenportal und Webhook-Verarbeitung

app/stripe.php: minimaler REST-Client fuer die Stripe-API auf Basis von
PHP-Streams statt eines vendorten SDKs - diese PHP-Installation hat keine
curl-Extension, Streams sind zudem portabler und brauchen kein
composer.json.

- Fuer jeden bezahlten Tarif per API ein Stripe-Produkt mit monatlichem
  Preis angelegt, referenziert ueber einen stabilen lookup_key statt
  hartcodierter Price-ID; Erstellung ist idempotent.
- abo-upgrade.php: erstellt eine Stripe-Checkout-Session fuer den
  gewaehlten Tarif, tenant_id/plan_code als Metadaten auf Session UND
  Subscription (damit spaetere Subscription-Events zuordenbar bleiben).
- abo-portal.php: oeffnet das Stripe Customer Portal fuer bestehende
  Kunden (Zahlungsmittel/Kuendigung, ohne eigene UI dafuer).
- stripe-webhook.php: verifiziert die Stripe-Signature per HMAC-SHA256
  mit Zeitstempel-Toleranz, verarbeitet checkout.session.completed,
  customer.subscription.updated/.deleted, invoice.paid/.payment_failed
  und haelt tenant_billing aktuell. Bewusst kein CSRF-/Login-Check
  (Stripe ruft unauthentifiziert auf), stattdessen ausschliesslich
  Signaturpruefung als Echtheitsnachweis.
- mandant-einstellungen.php: echter "Jetzt upgraden"-Button (Stripe
  Checkout) sowie "Zahlungsmethode verwalten/Abo kuendigen" (Customer
  Portal), sobald ein Stripe-Kunde existiert.

Live getestet (Stripe-Testmodus, kein echtes Geld): voller Checkout-Flow
bis zum echten Redirect auf checkout.stripe.com, Webhook-Verarbeitung
durch selbst erzeugte, korrekt signierte Test-Events (da diese
Dev-Umgebung keine oeffentlich erreichbare URL fuer echte Stripe-
Zustellung hat) inklusive Ablehnung falscher Signaturen, Customer Portal
mit echtem per API angelegtem Test-Kunden. Alle Regressionstests
weiterhin gruen (36 Seiten HTTP-Smoke, Golden Master, M8-Isolation/
Rollenmatrix).

Noch offen: echten Webhook-Endpunkt auf die Produktions-Domain eintragen,
sobald diese feststeht; Wechsel auf Live-Keys; Dolibarr-Sync (Phase 3).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/billing.php

@@ -3,25 +3,48 @@
 declare(strict_types=1);
 
 require_once __DIR__ . '/bootstrap.php';
+require_once __DIR__ . '/stripe.php';
 
 /**
  * Single source of truth for the pricing tiers, matching preise.php and
  * agb.php. max_participants = null means "no upper limit defined, contact
- * us" (the "auf Anfrage" tier).
+ * us" (the "auf Anfrage" tier). stripe_lookup_key is null for tiers that
+ * have no Stripe Price (free and enterprise are never checked out through
+ * Stripe: free needs no payment, enterprise is arranged manually).
  *
- * @return array<string, array{label: string, max_participants: ?int, price_cents: int}>
+ * @return array<string, array{label: string, max_participants: ?int, price_cents: int, stripe_lookup_key: ?string}>
  */
 function billing_plans(): array
 {
     return [
-        'free' => ['label' => 'Kostenlos', 'max_participants' => 10, 'price_cents' => 0],
-        'basic' => ['label' => 'Basic', 'max_participants' => 25, 'price_cents' => 399],
-        'plus' => ['label' => 'Plus', 'max_participants' => 50, 'price_cents' => 799],
-        'pro' => ['label' => 'Pro', 'max_participants' => 150, 'price_cents' => 1299],
-        'enterprise' => ['label' => 'Enterprise (auf Anfrage)', 'max_participants' => null, 'price_cents' => 0],
+        'free' => ['label' => 'Kostenlos', 'max_participants' => 10, 'price_cents' => 0, 'stripe_lookup_key' => null],
+        'basic' => ['label' => 'Basic', 'max_participants' => 25, 'price_cents' => 399, 'stripe_lookup_key' => 'kaffeeliste_basic'],
+        'plus' => ['label' => 'Plus', 'max_participants' => 50, 'price_cents' => 799, 'stripe_lookup_key' => 'kaffeeliste_plus'],
+        'pro' => ['label' => 'Pro', 'max_participants' => 150, 'price_cents' => 1299, 'stripe_lookup_key' => 'kaffeeliste_pro'],
+        'enterprise' => ['label' => 'Enterprise (auf Anfrage)', 'max_participants' => null, 'price_cents' => 0, 'stripe_lookup_key' => null],
     ];
 }
 
+/**
+ * Resolves the Stripe Price id for a plan by its lookup_key. Returns null
+ * for plans without a Stripe price (free, enterprise) or on API failure.
+ */
+function billing_stripe_price_id(string $planCode): ?string
+{
+    $plans = billing_plans();
+    $lookupKey = $plans[$planCode]['stripe_lookup_key'] ?? null;
+    if ($lookupKey === null) {
+        return null;
+    }
+
+    $result = stripe_request('GET', 'prices', ['lookup_keys' => [$lookupKey], 'active' => 'true']);
+    if ($result['ok'] && !empty($result['data']['data'][0]['id'])) {
+        return (string)$result['data']['data'][0]['id'];
+    }
+
+    return null;
+}
+
 /**
  * Returns the cheapest plan whose participant limit still covers
  * $activeParticipantCount. Used both to show customers which plan they
@@ -72,6 +95,40 @@ function billing_fetch_or_init(PDO $pdo, int $tenantId): array
     return $row;
 }
 
+function billing_update(PDO $pdo, int $tenantId, array $fields): void
+{
+    billing_fetch_or_init($pdo, $tenantId);
+
+    $setClauses = [];
+    $params = [];
+    foreach ($fields as $column => $value) {
+        $setClauses[] = "{$column} = ?";
+        $params[] = $value;
+    }
+    $params[] = $tenantId;
+
+    $pdo->prepare('UPDATE tenant_billing SET ' . implode(', ', $setClauses) . ' WHERE tenant_id = ?')
+        ->execute($params);
+}
+
+function billing_find_tenant_id_by_stripe_customer(PDO $pdo, string $stripeCustomerId): ?int
+{
+    $stmt = $pdo->prepare('SELECT tenant_id FROM tenant_billing WHERE stripe_customer_id = ?');
+    $stmt->execute([$stripeCustomerId]);
+    $tenantId = $stmt->fetchColumn();
+
+    return $tenantId !== false ? (int)$tenantId : null;
+}
+
+function billing_find_tenant_id_by_stripe_subscription(PDO $pdo, string $stripeSubscriptionId): ?int
+{
+    $stmt = $pdo->prepare('SELECT tenant_id FROM tenant_billing WHERE stripe_subscription_id = ?');
+    $stmt->execute([$stripeSubscriptionId]);
+    $tenantId = $stmt->fetchColumn();
+
+    return $tenantId !== false ? (int)$tenantId : null;
+}
+
 /**
  * Compares the tenant's currently booked plan against what their active
  * participant count actually requires. Purely informational until the