feat: run release checks in push-to-release PRs

We want to have as much confidence as we can get that the release code
is sound.

Author: pheuberger

.github/workflows/release-checks.yml

@@ -0,0 +1,31 @@
+name: Release Pipeline
+on:
+  pull_request:
+    branches: [ release ]
+  push:
+    branches: [ release ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: '9.2.0'
+
+      - name: Install dependencies
+        run: pnpm install
+
+      - name: Run tests
+        run: pnpm test
+
+      - name: Build frontend
+        run: pnpm build