Configure docker-compose for Coolify/Traefik deployment

Replace the host-port-published nginx service with one that joins an
external Docker network and routes entirely through Traefik labels
(HTTP->HTTPS redirect, TLS router, security-headers middleware), matching
the pattern used by other Coolify-deployed projects. Add .env.example for
TRAEFIK_ENABLED/TRAEFIK_HOST/NETWORK_NAME and document the new deploy flow
and local-preview alternative in the README.

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

.gitignore

@@ -32,6 +32,7 @@ yarn-error.log*
 
 # env files (can opt-in for committing if needed)
 .env*
+!.env.example
 
 # vercel
 .vercel