Harte Teilnehmer-Obergrenze je Tarif statt automatischem Stufenwechsel

Statt automatisch hochzustufen, blockiert das Anlegen/Reaktivieren
weiterer aktiver Mitglieder, sobald das Limit des gebuchten Tarifs
erreicht ist (Neuanlage, Bearbeiten mit Statuswechsel, Aktivieren-
Button). mitarbeiterverwalten.php zeigt die aktuelle Auslastung an.
Neuer Regressionstest scripts/check-billing-capacity.php deckt alle
drei Enforcement-Stellen ab.

app/billing.php

@@ -129,6 +129,34 @@ function billing_find_tenant_id_by_stripe_subscription(PDO $pdo, string $stripeS
     return $tenantId !== false ? (int)$tenantId : null;
 }
 
+/**
+ * Hard cap enforcement: whether the tenant's current plan still has room
+ * for $additional more active participants. There is no automatic plan
+ * upgrade - tenants that outgrow their plan must upgrade themselves
+ * (mandant-einstellungen.php) before adding/reactivating more members.
+ */
+function billing_has_capacity_for(PDO $pdo, int $tenantId, int $additional = 1): bool
+{
+    $billing = billing_fetch_or_init($pdo, $tenantId);
+    $max = billing_plans()[$billing['plan_code']]['max_participants'] ?? null;
+    if ($max === null) {
+        return true;
+    }
+
+    return billing_count_active_participants($pdo, $tenantId) + $additional <= $max;
+}
+
+function billing_capacity_error_message(PDO $pdo, int $tenantId): string
+{
+    $billing = billing_fetch_or_init($pdo, $tenantId);
+    $plan = billing_plans()[$billing['plan_code']] ?? null;
+    $label = $plan['label'] ?? $billing['plan_code'];
+    $max = $plan['max_participants'] ?? null;
+
+    return "Euer aktueller Tarif „{$label}\" erlaubt maximal {$max} aktive Mitglieder. "
+        . 'Bitte upgradet unter Mandant-Einstellungen, um mehr Mitglieder freizuschalten.';
+}
+
 /**
  * Compares the tenant's currently booked plan against what their active
  * participant count actually requires. Purely informational until the