feat(bookwyrm): production-ready runtime + Redis wiring

- Dockerfile: build & install gunicorn wheels
- compose: run initdb before start; use `python -m gunicorn`
- env: add POSTGRES_* and BookWyrm Redis aliases (BROKER/ACTIVITY/CACHE) + CACHE_URL
- vars: add cache URL, DB indices, and URL aliases for Redis

Ref: https://chatgpt.com/share/68b7492b-3200-800f-80c4-295bc3233d68

roles/web-app-bookwyrm/templates/docker-compose.yml.j2

@@ -6,7 +6,8 @@
       bash -lc '
         python manage.py migrate --noinput &&
         python manage.py collectstatic --noinput &&
-        gunicorn bookwyrm.wsgi:application --bind 0.0.0.0:{{ container_port }}
+        (python manage.py initdb || true) &&
+        python -m gunicorn bookwyrm.wsgi:application --bind 0.0.0.0:{{ container_port }}
       '
     build:
       context: .