Add calendar app — shared events calendar with departments, staff tagging, bank holidays and two-way phone sync

Fastify/Postgres backend + React frontend matching the stack's app conventions, plus
a hand-rolled RFC 4791 CalDAV server (caldav-adapter turned out Koa-only in practice)
so calendars subscribe as genuine two-way sync in Apple/Google/Outlook. v1 scope:
multiple colour-coded calendars, department/staff event tagging via live Workforce
lookups, month/week/day/list views, dashboard, file attachments, activity log, and
an auto-synced UK bank holidays calendar. Verified end-to-end locally against real
Postgres: REST CRUD, CalDAV discovery/PROPFIND/REPORT/PUT/sync-collection, all-day
date handling, system-calendar write protection, and activity logging across both
the web and CalDAV write paths.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "hnf-calendar-backend",
+  "version": "1.0.0",
+  "type": "module",
+  "scripts": {
+    "start": "node src/index.js",
+    "dev": "node --watch src/index.js"
+  },
+  "dependencies": {
+    "@fastify/cookie": "^9.4.0",
+    "@fastify/cors": "^9.0.1",
+    "@fastify/multipart": "^8.3.0",
+    "@fastify/static": "^7.0.4",
+    "@xmldom/xmldom": "^0.8.10",
+    "bcryptjs": "^2.4.3",
+    "fastify": "^4.28.1",
+    "ical.js": "^2.1.0",
+    "ics": "^3.8.1",
+    "jose": "^5.9.6",
+    "pg": "^8.13.1"
+  }
+}