build: add production-parity staging environment on Docker

Layer a staging overlay (docker-compose.staging.yml) on the base compose file:
same Linux images and Production runtime as prod, differing only in the dev
banner, capped sync, shifted ports (18080/18081), and an isolated project
namespace so it never touches prod data. Wire -Staging into deploy/up.ps1 and
deploy/down.ps1, add .env.staging.example, and track that template in git.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -54,6 +54,7 @@ lpt[1-9].*
 *.code-workspace
 .env.*
 !.env.example
+!.env.staging.example
 .next/
 dist/
 build/