Add backend tests to CI, triggered only on backend/ changes

Compares changed files to detect backend modifications. On pushes
to main, always runs if backend files changed. On PRs, compares
against the base branch. Skips test setup entirely when only
frontend or other files are modified.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ambv

.github/workflows/ci.yml

@@ -40,3 +40,34 @@ jobs:
             echo "::error::Please run pip-compile to regenerate lockfiles. See README for instructions."
             exit 1
           fi
+
+  backend-tests:
+    name: Backend tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: Check for backend changes
+        id: changes
+        run: |
+          if [ "${{ github.event_name }}" = "push" ]; then
+            CHANGED=$(git diff --name-only HEAD~1 HEAD 2>/dev/null || echo "backend/")
+          else
+            CHANGED=$(git diff --name-only ${{ github.event.pull_request.base.sha }} ${{ github.event.pull_request.head.sha }})
+          fi
+          if echo "$CHANGED" | grep -q '^backend/'; then
+            echo "backend=true" >> "$GITHUB_OUTPUT"
+          fi
+      - uses: actions/setup-python@v5
+        if: steps.changes.outputs.backend == 'true'
+        with:
+          python-version: "3.13"
+          cache: pip
+          cache-dependency-path: backend/requirements-dev.txt
+      - name: Run tests
+        if: steps.changes.outputs.backend == 'true'
+        working-directory: backend
+        run: |
+          pip install -r requirements-dev.txt
+          python -m pytest tests/ -v --cov=app --cov-report=term-missing