Add AI cost insights dashboard; rota-informed forecast method

AI Insights: a daily Claude-generated wage cost briefing covering
month-to-date pace vs budget, prior-month/prior-year comparison,
rota-vs-actual variance by department, a rota-informed forecast to
month-end, employee-level anomalies, and wage cost as a % of revenue.
Runs on a configurable daily schedule or on demand, gated by a manual
5-minute rate limit and a daily token budget. Uses the Anthropic key
configured centrally in Portal → Settings → Integrations.

Also includes the rota-vs-repeat-pattern forecast method (published/
draft rota tiers with same-weekday fallback) already built into the
Weekly/Monthly views, and adds a .gitignore for node_modules/dist.

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

backend/package.json

@@ -7,6 +7,7 @@
     "dev": "node --watch src/index.js"
   },
   "dependencies": {
+    "@anthropic-ai/sdk": "^0.32.1",
     "@fastify/cookie": "^9.4.0",
     "@fastify/cors": "^9.0.1",
     "fastify": "^4.28.1",