Rework README and docker-compose comments to document the deliberate
chicken-and-egg escape: the `init` sidecar now only runs `migrate` and
`load_library_types`, leaving `setup_neo4j_indexes` as a manual step
after the system embedding model is configured in `/admin/`. This
avoids making `app` unreachable on first boot when no embedding model
row exists yet, while preserving loud failure on dimension mismatch.