chore: Bump backend version to 1.6.0

Harmonize backend version with WordPress plugin version so admin UI displays correct version.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "dutchie-menus-backend",
-  "version": "1.5.1",
+  "version": "1.6.0",
   "description": "Backend API for Dutchie Menus scraper and management",
   "main": "dist/index.js",
   "scripts": {