ci: install Python deps before Playwright E2E to fix missing Flask in CI

Author: patchmemory

.github/workflows/ci.yml

@@ -37,6 +37,16 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Install Python dependencies (app + dev)
+        run: |
+          python -m pip install --upgrade pip
+          pip install -e .[dev]
+
       - name: Set up Node.js
         uses: actions/setup-node@v4
         with: