📦️(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/installation/compose.md

@@ -260,7 +260,7 @@ The collaboration server never runs DDL itself, so its schema has to be created
 before it starts:
 
 ```bash
-docker compose run --rm yhub npm run init-db
+docker compose run --rm yhub yarn init-db
 ```
 
 It creates the `yhub` database when it is missing, and every table the version
@@ -319,7 +319,7 @@ docker compose run --rm backend python manage.py migrate
 ```
 and the collaboration server's, which is the same command as at install time:
 ```bash
-docker compose run --rm yhub npm run init-db
+docker compose run --rm yhub yarn init-db
 ```
 
 ## Upgrading from a release without the collaboration server