refactor: use $DOMAIN env var instead of hardcoded lazyworkhorse.net

Replace all hardcoded lazyworkhorse.net references in compose files
with ${DOMAIN} variable substitution. Create .env.production and
.env.staging environment files. Update Makefile with ENV selection
(--env-file support) and staging/production targets.

Changes:
- All 13 compose YAML files: lazyworkhorse.net -> ${DOMAIN}
- New .env.production (DOMAIN=lazyworkhorse.net)
- New .env.staging (DOMAIN=staging.lazyworkhorse.net)
- Makefile: ENV var, --env-file flag, staging/production targets
- Gitea redirect regex updated for variable substitution
- CI workflow checkout URLs left hardcoded (infrastructure refs)
- Dockerfile SSH host refs left hardcoded (infrastructure refs)

Deploy: make ENV=staging all_up  or  make staging
        make ENV=production all_up  or  make production

vpn/compose.yml

@@ -11,7 +11,7 @@ services:
       - NET_ADMIN
       - SYS_MODULE
     environment:
-      - WG_HOST=vpn.lazyworkhorse.net
+      - WG_HOST=vpn.${DOMAIN}
       - PASSWORD=${WG_PASSWORD}
       - WG_PORT=51820
       - WG_DEFAULT_ADDRESS=10.8.0.x