📦️(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`.

.github/workflows/impress.yml

@@ -236,7 +236,7 @@ jobs:
 
       - name: Install the collaboration server
         working-directory: src/yhub-server
-        run: npm ci --omit=dev
+        run: yarn install --frozen-lockfile --production
 
       # yhub ships its own DDL and creates the database as well, so this needs
       # the dependencies installed above — hence its place after them
@@ -244,7 +244,7 @@ jobs:
         working-directory: src/yhub-server
         env:
           POSTGRES: postgres://dinum:pass@localhost:5432/yhub
-        run: npm run init-db
+        run: yarn init-db
 
       - name: Start the backend for the collaboration server to authenticate against
         env: