CI: Use npm ci instead of npm install

`npm ci` is recommended instead of `npm install` for automated builds.
Its main advantage is the `package-lock.json` consistency check: The
command fails if the lock file doesn't match `package.json`.

.github/workflows/ci.yml

@@ -27,12 +27,12 @@ jobs:
 
       - name: Install
         if: ${{ matrix.flavor == 'dev'}}
-        run: npm install
+        run: npm ci
         working-directory: ${{ matrix.flavor }}/backend
 
       - name: Install (Prod dependencies only)
         if: ${{ matrix.flavor == 'prod'}}
-        run: npm install --prod
+        run: npm ci --prod
         working-directory: ${{ matrix.flavor }}/backend
 
       - name: Lint 
@@ -67,13 +67,13 @@ jobs:
           registry-url: 'https://registry.npmjs.org'
 
       - name: Install (Prod dependencies only)
-        run: npm install --prod
+        run: npm ci --prod
         if: ${{ matrix.flavor == 'prod'}}
         working-directory: ${{ matrix.flavor }}/frontend
 
       - name: Install
         if: ${{ matrix.flavor == 'dev'}}
-        run: npm install
+        run: npm ci
         working-directory: ${{ matrix.flavor }}/frontend
 
       - name: Lint