feat: revocable read-only API keys for machine access

Integrations such as the Wabenhain shop sync no longer need a service
account with a password (and without MFA). An admin creates a key under
Verwaltung > Automatisierung > API-Schlüssel.

- Key format ack_<8>_<43>; only its SHA-256 hash is stored, the key is
  shown exactly once. Optional expiry, "last used" timestamp, revocation
  takes effect immediately; create/revoke are audit-logged without the key.
- A key is not a user: JwtAuthGuard sets request.apiKey, PermissionsGuard
  admits it only to routes whose permission is among its scopes and never
  to user-only routes. Scopes come from a short allowlist (shop-sync:read).
- Managing keys requires the new permission api-keys:manage.
- Keys are excluded from backups on purpose; a restore means issuing a new
  key.

Additive migration (new table).

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

ApiCore-Backend/prisma/migrations/20260927231113_add_api_keys/migration.sql

@@ -0,0 +1,24 @@
+-- CreateTable
+CREATE TABLE "ApiKey" (
+    "id" TEXT NOT NULL,
+    "name" TEXT NOT NULL,
+    "prefix" TEXT NOT NULL,
+    "keyHash" TEXT NOT NULL,
+    "scopes" TEXT[],
+    "createdByUserId" TEXT,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "lastUsedAt" TIMESTAMP(3),
+    "expiresAt" TIMESTAMP(3),
+    "revokedAt" TIMESTAMP(3),
+
+    CONSTRAINT "ApiKey_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "ApiKey_keyHash_key" ON "ApiKey"("keyHash");
+
+-- CreateIndex
+CREATE INDEX "ApiKey_revokedAt_idx" ON "ApiKey"("revokedAt");
+
+-- AddForeignKey
+ALTER TABLE "ApiKey" ADD CONSTRAINT "ApiKey_createdByUserId_fkey" FOREIGN KEY ("createdByUserId") REFERENCES "User"("id") ON DELETE SET NULL ON UPDATE CASCADE;