Run the pod provider from images with make start, and fix what a fresh stack exposed

- docker-compose-dev.yml: the redis host port is overridable (REDIS_PORT in the root
  .env, 6379 by default as in the sibling apps), and the pod provider, in host network
  mode, follows it for its queue and OIDC redis. All of ./data/fuseki is mounted at the
  image's SEMAPPS_FUSEKI_BASE, so account deletion removes the backups and, 24 h later,
  the secure dataset's files (it answered 500 without the mount).
- backend: GET /app served the app actor snapshot the mixin takes before it creates or
  replaces the access-need group, so a dataset's first boot lacked the link until a
  restart; the app service refreshes the snapshot from the store at the end of started().
- e2e smoke: the post-install poll sends Accept: application/ld+json; the pod answers 500
  to a container read in any other format.
- Docs: cookbook (Redis port, mounts, hot reload, dataset-named accounts, fresh-boot
  note), deployment note, env examples; claims about a parallel race are no longer stated
  as fact.

.env.example

@@ -45,6 +45,12 @@ NODE_ENV=
 
 # Le backend lit son propre backend/.env (voir backend/.env.example).
 
+# ── Infrastructure locale (`make start`) ───────────────────────────────────
+# docker compose lit aussi ce fichier. Port hôte du Redis de docker-compose-dev.yml,
+# vide = 6379. Le changer si un autre Redis occupe ce port, et reporter la même
+# valeur dans SEMAPPS_QUEUE_SERVICE_URL de backend/.env.
+REDIS_PORT=
+
 # ── Frontend / Backend coupling ────────────────────────────────────────────
 # URL du backend telle que vue par le navigateur.
 VITE_BACKEND_URL=http://localhost:3005