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-Backend/src/honey-batches/dto/set-honey-batch-container-allocation.dto.ts

@@ -0,0 +1,9 @@
+import { ApiProperty } from '@nestjs/swagger';
+import { IsNumber, IsPositive } from 'class-validator';
+
+export class SetHoneyBatchContainerAllocationDto {
+  @ApiProperty({ description: 'Menge in kg in diesem Behälter' })
+  @IsNumber()
+  @IsPositive()
+  amount!: number;
+}