feat: add Nginx to Caddy migration toolkit with scripts and usage guide

setup/nginx-to-caddy/CUTOVER_CHECKLIST.md

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+# Cutover Checklist
+
+## Pre-cutover
+- [ ] `nginx -T` snapshot captured (`output/nginx-full.conf`)
+- [ ] Generated Caddyfile reviewed
+- [ ] `conversion-warnings.txt` reviewed and resolved for canary site
+- [ ] `validate_caddy.sh` passes
+- [ ] DNS TTL lowered for canary domain
+
+## Canary
+- [ ] One subdomain switched to Caddy
+- [ ] HTTPS cert valid
+- [ ] UI and API calls work
+- [ ] Websocket/live components work
+- [ ] Access/error logs clean
+
+## Full rollout
+- [ ] Remaining subdomains switched in batches
+- [ ] Monitor 24h
+- [ ] Keep Nginx rollback config archived
+
+## Rollback trigger
+- [ ] TLS failures
+- [ ] High 4xx/5xx
+- [ ] Broken websockets/realtime components
+- [ ] Unexpected redirects or auth loops