feat(ui): article stock overview and production batch editing

- New article page /products/[id]: remaining stock summed over all
  production batches (or bottlings for honey), each batch linked to its
  own page for withdrawals and counts; "Bestand ansehen" in the list.
- Production batches can be corrected (lot number, date, quantity,
  best-before date, note) via PATCH; the article stays fixed.

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

ApiCore-Frontend/src/lib/production-batches/api.ts

@@ -3,6 +3,7 @@ import {
   CreateProductionBatchInput,
   ProductionBatch,
   ProductionBatchFilter,
+  UpdateProductionBatchInput,
 } from "./types";
 
 export function fetchProductionBatches(
@@ -29,6 +30,16 @@ export function createProductionBatch(
   });
 }
 
+export function updateProductionBatch(
+  id: string,
+  input: UpdateProductionBatchInput,
+): Promise<ProductionBatch> {
+  return apiFetch<ProductionBatch>(`/production-batches/${id}`, {
+    method: "PATCH",
+    body: JSON.stringify(input),
+  });
+}
+
 export function deleteProductionBatch(id: string): Promise<{ id: string }> {
   return apiFetch<{ id: string }>(`/production-batches/${id}`, {
     method: "DELETE",

ApiCore-Frontend/src/lib/production-batches/types.ts

@@ -20,6 +20,15 @@ export interface CreateProductionBatchInput {
   notes?: string;
 }
 
+export interface UpdateProductionBatchInput {
+  lotNumber?: string;
+  date?: string;
+  quantity?: number;
+  /** `null` removes the date, `undefined` leaves it untouched. */
+  bestBeforeDate?: string | null;
+  notes?: string;
+}
+
 export interface ProductionBatchFilter {
   productId?: string;
 }