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>

deploy/down.ps1

@@ -5,13 +5,22 @@
   ./deploy/down.ps1
   ./deploy/down.ps1 -Volumes
 #>
-param([switch]$Volumes)
+param([switch]$Volumes, [switch]$Staging)
 
 $ErrorActionPreference = 'Stop'
-$root    = Split-Path -Parent $PSScriptRoot
-$envFile = Join-Path $PSScriptRoot '.env'
+$root = Split-Path -Parent $PSScriptRoot
 
-$composeArgs = @('compose', '--env-file', $envFile, 'down')
+if ($Staging) {
+  $envFile      = Join-Path $PSScriptRoot '.env.staging'
+  $composeFiles = @('-f', 'docker-compose.yml', '-f', 'docker-compose.staging.yml')
+  $project      = @('-p', 'inboxintel-staging')
+} else {
+  $envFile      = Join-Path $PSScriptRoot '.env'
+  $composeFiles = @()
+  $project      = @()
+}
+
+$composeArgs = @('compose') + $project + @('--env-file', $envFile) + $composeFiles + @('down')
 if ($Volumes) { $composeArgs += '--volumes' }
 
 Push-Location $root