chore(deploy): add same-host staging stack next to production

dev deploys to /opt/dukang-staging (819x, *-test domains); main deploys to production. Staging uses full Mock with shared WeChat app ids.

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -8,6 +8,8 @@ server/dukang-api/.env
 server/dukang-api/.env.development
 server/dukang-api/.env.production
 server/dukang-api/.env.production.local
+server/dukang-api/.env.staging
+server/dukang-api/.env.staging.local
 deploy/deploy.env
 .DS_Store
 coverage/