feat: allow a honey batch to be split across multiple storage containers

A batch that doesn't fit in one container (e.g. 125 kg across 5 x 25 kg
buckets) had no way to be represented - HoneyBatch.storageContainerId
is a single nullable FK. Adds HoneyBatchContainerAllocation as an
additive join table (container + amount per batch), independent of the
existing single-container/relocate() flow which still covers the
simple one-container case unchanged.

- GET/PUT/DELETE /honey-batches/:id/containers[/:containerId], reusing
  the existing honey-batches:relocate permission.
- Assigning a container is an upsert (@@unique on batch+container), and
  the total assigned is validated against the batch's own amount when
  known.
- New "Aufteilung auf mehrere Behälter" card on the honey batch detail
  page, alongside (not replacing) the existing storage card.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

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

@@ -3,6 +3,7 @@ import {
   CreateHoneyBatchInput,
   CreateHoneyMeasurementInput,
   HoneyBatch,
+  HoneyBatchContainerAllocation,
   HoneyBatchRelocation,
   HoneyBatchStatus,
   RelocateHoneyBatchInput,
@@ -69,3 +70,29 @@ export function relocateHoneyBatch(
 export function fetchHoneyBatchRelocations(id: string): Promise<HoneyBatchRelocation[]> {
   return apiFetch<HoneyBatchRelocation[]>(`/honey-batches/${id}/relocations`);
 }
+
+export function fetchHoneyBatchContainerAllocations(
+  id: string,
+): Promise<HoneyBatchContainerAllocation[]> {
+  return apiFetch<HoneyBatchContainerAllocation[]>(`/honey-batches/${id}/containers`);
+}
+
+export function setHoneyBatchContainerAllocation(
+  id: string,
+  containerId: string,
+  amount: number,
+): Promise<HoneyBatchContainerAllocation> {
+  return apiFetch<HoneyBatchContainerAllocation>(
+    `/honey-batches/${id}/containers/${containerId}`,
+    { method: "PUT", body: JSON.stringify({ amount }) },
+  );
+}
+
+export function removeHoneyBatchContainerAllocation(
+  id: string,
+  containerId: string,
+): Promise<void> {
+  return apiFetch<void>(`/honey-batches/${id}/containers/${containerId}`, {
+    method: "DELETE",
+  });
+}