feat(stock): keep shop stock in sync with ApiCore, write sales back

ApiCore now owns the remaining stock per batch; the shop sells from it
and reports every paid sale and every return back.

- Outbox apicore_stock_events, written in the same transaction as the
  status change: paid -> SALE, cancelled-after-payment / refunded return
  -> RETURN onto the batches of the sale, reopened paid order -> SALE.
  Whether a return is due is decided by the open sales in the outbox,
  so unpaid orders never book and nothing is booked twice.
- Delivery right after the response (next/server after()) plus CronJob
  wabenhain-apicore-stock every 10 minutes with 1/5/15/60 min backoff;
  ApiCore deduplicates by event id. Rejected bookings stop and show up
  on the dashboard.
- Reconcile: shop stock = ApiCore remaining - unconfirmed bookings -
  reservations of unpaid orders, in a serializable transaction like the
  checkout. Triggered after bookings, by the signed ApiCore webhook
  stock.changed (ids only, numbers are fetched) and by the sync button.
- Manufactured goods (candles, propolis products, ...): one shop product
  per ApiCore article via Product.apicoreProductId, drafts created by the
  sync, existing products linkable in the product form; new category
  SONSTIGES. Stock field is read-only for linked products.
- OrderItem.apicoreAllocations keeps which lots were sold (recalls).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

k8s/02-secret.example.yaml

@@ -109,6 +109,12 @@ stringData:
   # Konfigurationsfehler. APICORE_API_URL steht in 01-configmap.yaml.
   APICORE_API_KEY: ""
 
+  # Secret des ApiCore-Webhooks "stock.changed" (ApiCore -> Automatisierung ->
+  # Webhooks, Ziel https://<shop>/api/apicore/webhook). Damit kommen
+  # Entnahmen aus ApiCore sofort im Shop an; leer = nur der CronJob gleicht
+  # ab (alle 10 Minuten).
+  APICORE_WEBHOOK_SECRET: ""
+
   # DHL-Versandmarken (Admin -> Bestellung -> "DHL-Versandmarke"), siehe
   # .env.example fuer die vollstaendige Einrichtung. Leer = der Knopf meldet
   # "noch nicht konfiguriert", der Rest des Shops ist unberuehrt. Ob Sandbox