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]>

src/components/admin-form.tsx

@@ -15,6 +15,7 @@ export function AdminField({
   required = false,
   min,
   hint,
+  readOnly = false,
 }: {
   name: string;
   label: string;
@@ -25,6 +26,8 @@ export function AdminField({
   min?: number;
   /** Kurzer Zusatz unter dem Feld, statt ihn in die Beschriftung zu packen. */
   hint?: string;
+  /** Wird angezeigt und mitgeschickt, laesst sich aber nicht aendern. */
+  readOnly?: boolean;
 }) {
   return (
     <label className="block text-sm">
@@ -36,7 +39,8 @@ export function AdminField({
         placeholder={placeholder}
         required={required}
         min={min}
-        className={inputClass}
+        readOnly={readOnly}
+        className={readOnly ? `${inputClass} bg-ink/5 text-ink/70` : inputClass}
       />
       {hint ? <span className="mt-1 block text-xs text-ink/65">{hint}</span> : null}
     </label>