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>

.env.staging.example

@@ -0,0 +1,17 @@
+# Copy to deploy/.env.staging and fill in. Used by:
+#   docker compose -p inboxintel-staging --env-file deploy/.env.staging \
+#     -f docker-compose.yml -f docker-compose.staging.yml up
+# (or ./deploy/up.ps1 -Staging). Kept separate from deploy/.env (production) so
+# staging can never touch prod credentials, DB, or Google project.
+POSTGRES_PASSWORD=change-me-staging
+
+# Use a SEPARATE Google OAuth client for staging with redirect URI:
+#   http://localhost:18081/signin-google
+GOOGLE_CLIENT_ID=
+GOOGLE_CLIENT_SECRET=
+
+AI_MODE=Disabled
+FRONTEND_ORIGIN=http://localhost:18081
+
+# Staging caps the initial mailbox sync so rehearsals are fast.
+MAX_MESSAGES=2000