Add baseline CI workflow

.github/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Install lightweight validation tools
+        run: python -m pip install --upgrade pip ruff
+
+      - name: Compile Python source
+        run: python -m py_compile app.py
+
+      - name: Run Ruff lint check
+        # The existing monolith has legacy lint findings. Keep reporting them
+        # while allowing the baseline check to pass during the refactor.
+        run: ruff check . --exclude venv,models --output-format=github --exit-zero