📦️(yhub) switch yhub-server to yarn

Every other package in this repository is installed
with yarn; yhub-server was the only one on npm,
with its own package-lock.json.
To ensure consistency it now uses yarn like the rest
of the project.

package-lock.json is replaced by yarn.lock, a packageManager
field is added, and the Dockerfile, CI workflow, Makefile,
helm init-db job and the documentation move from
`npm ci` / `npm run init-db` to
`yarn install --frozen-lockfile` / `yarn init-db`.

documentation/examples/compose/compose.yaml

@@ -62,7 +62,7 @@ services:
 
   # The collaboration server: it serves everything under /collaboration/, the
   # websocket included, and holds the content of the documents. Its schema is
-  # not created when it starts — run `docker compose run --rm yhub npm run
+  # not created when it starts — run `docker compose run --rm yhub yarn
   # init-db` once before the first start, and again after every upgrade.
   yhub:
     image: lasuite/impress-yhub:latest