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/prisma/migrations/20260904150000_add_honey_batch_container_allocations/migration.sql

@@ -0,0 +1,23 @@
+-- CreateTable
+CREATE TABLE "HoneyBatchContainerAllocation" (
+    "id" TEXT NOT NULL,
+    "honeyBatchId" TEXT NOT NULL,
+    "storageContainerId" TEXT NOT NULL,
+    "amount" DOUBLE PRECISION NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "HoneyBatchContainerAllocation_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "HoneyBatchContainerAllocation_storageContainerId_idx" ON "HoneyBatchContainerAllocation"("storageContainerId");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "HoneyBatchContainerAllocation_honeyBatchId_storageContaine_key" ON "HoneyBatchContainerAllocation"("honeyBatchId", "storageContainerId");
+
+-- AddForeignKey
+ALTER TABLE "HoneyBatchContainerAllocation" ADD CONSTRAINT "HoneyBatchContainerAllocation_honeyBatchId_fkey" FOREIGN KEY ("honeyBatchId") REFERENCES "HoneyBatch"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "HoneyBatchContainerAllocation" ADD CONSTRAINT "HoneyBatchContainerAllocation_storageContainerId_fkey" FOREIGN KEY ("storageContainerId") REFERENCES "StorageContainer"("id") ON DELETE RESTRICT ON UPDATE CASCADE;