Deployability + docspec fix on top of 2.0.0+v6.0.4.

The backend and yhub RSA keys are now generated automatically on first deploy
into the `jwt_keys` volume (the `jwt-keys` service) and reused afterwards; no
manual secret insertion is needed. `SECRET_JWT_PK_VERSION` and
`SECRET_YHUB_JWT_PK_VERSION` are no longer used.

Docspec moves to the Rust `ghcr.io/docspec/api:1.21.4` (port 3000), which Docs
>= 5.7 expects; the archived Elixir `ghcr.io/docspecio/api` (port 4000) is
removed.

`yhub-valkey` is 9.1.3-alpine (from upstream 2.0.0); safe for this standalone,
no-auth deployment (see the valkey 9 release notes).