feat(backend): Webhooks mit HMAC-signierter Auslieferung (Modul 28)

WebhooksModule (WebhookSubscription: URL, Events, Secret, aktiv) mit
Event-Dispatcher, angehaengt an 10 Ereignisse in 8 bestehenden Services
(colony.created, inspection.created, queen.created, queen.replaced,
harvest.created, batch.created, batch.updated, product.stock_changed,
task.created, task.completed). Auslieferung ist best-effort und blockiert
die ausloesende Anfrage nicht bei Fehlern.

ApiCore-Backend/prisma/migrations/20260823044358_webhooks/migration.sql

@@ -0,0 +1,19 @@
+-- CreateTable
+CREATE TABLE "WebhookSubscription" (
+    "id" TEXT NOT NULL,
+    "url" TEXT NOT NULL,
+    "events" TEXT[],
+    "secret" TEXT NOT NULL,
+    "active" BOOLEAN NOT NULL DEFAULT true,
+    "createdByUserId" TEXT,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "WebhookSubscription_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "WebhookSubscription_active_idx" ON "WebhookSubscription"("active");
+
+-- AddForeignKey
+ALTER TABLE "WebhookSubscription" ADD CONSTRAINT "WebhookSubscription_createdByUserId_fkey" FOREIGN KEY ("createdByUserId") REFERENCES "User"("id") ON DELETE SET NULL ON UPDATE CASCADE;