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

backup/compose.yml

@@ -37,12 +37,12 @@ services:
   #   labels:
   #     - "traefik.enable=true"
   #     # 1. HTTP to HTTPS Redirect
-  #     - "traefik.http.routers.kopia-http.rule=Host(`backup.lazyworkhorse.net`)"
+  #     - "traefik.http.routers.kopia-http.rule=Host(`backup.${DOMAIN}`)"
   #     - "traefik.http.routers.kopia-http.entrypoints=web"
   #     - "traefik.http.routers.kopia-http.middlewares=redirect-to-https@docker"
   #     
   #     # 2. HTTPS Configuration
-  #     - "traefik.http.routers.kopia.rule=Host(`backup.lazyworkhorse.net`)"
+  #     - "traefik.http.routers.kopia.rule=Host(`backup.${DOMAIN}`)"
   #     - "traefik.http.routers.kopia.entrypoints=websecure"
   #     - "traefik.http.routers.kopia.tls=true"
   #     - "traefik.http.routers.kopia.tls.certresolver=njalla"
@@ -81,12 +81,12 @@ services:
     labels:
       - "traefik.enable=true"
       # 1. HTTP to HTTPS Redirect
-      - "traefik.http.routers.restic-browser-http.rule=Host(`backup.lazyworkhorse.net`)"
+      - "traefik.http.routers.restic-browser-http.rule=Host(`backup.${DOMAIN}`)"
       - "traefik.http.routers.restic-browser-http.entrypoints=web"
       - "traefik.http.routers.restic-browser-http.middlewares=redirect-to-https@docker"
       
       # 2. HTTPS Configuration
-      - "traefik.http.routers.restic-browser.rule=Host(`backup.lazyworkhorse.net`)"
+      - "traefik.http.routers.restic-browser.rule=Host(`backup.${DOMAIN}`)"
       - "traefik.http.routers.restic-browser.entrypoints=websecure"
       - "traefik.http.routers.restic-browser.tls=true"
       - "traefik.http.routers.restic-browser.tls.certresolver=njalla"