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

tak/compose.yml

@@ -74,12 +74,12 @@ services:
       - "traefik.docker.network=traefik-net"
 
       # HTTP -> HTTPS Redirect
-      - "traefik.http.routers.fts-ui-http.rule=Host(`tak.lazyworkhorse.net`)"
+      - "traefik.http.routers.fts-ui-http.rule=Host(`tak.${DOMAIN}`)"
       - "traefik.http.routers.fts-ui-http.entrypoints=web"
       - "traefik.http.routers.fts-ui-http.middlewares=redirect-to-https"
 
       # HTTPS Router
-      - "traefik.http.routers.fts-ui-https.rule=Host(`tak.lazyworkhorse.net`)"
+      - "traefik.http.routers.fts-ui-https.rule=Host(`tak.${DOMAIN}`)"
       - "traefik.http.routers.fts-ui-https.entrypoints=websecure"
       - "traefik.http.routers.fts-ui-https.tls=true"
       - "traefik.http.routers.fts-ui-https.tls.certresolver=njalla"