Replace GitLab CI with Gitea workflows

The GitLab deploy job has no replacement yet; deploy with bin/deploy by hand.

.gitea/workflows/tests.yml

@@ -0,0 +1,36 @@
+name: Tests
+
+on:
+  pull_request:
+  workflow_call:
+
+jobs:
+  build_frontend:
+    runs-on: ubuntu-latest
+    container: node:20-alpine
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build frontend
+        working-directory: frontend
+        run: |
+          npm ci
+          npm run build
+
+  tests:
+    runs-on: ubuntu-latest
+    container: nikolaik/python-nodejs:python3.11-nodejs20
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Run migrations
+        run: uv run --locked manage.py migrate
+      - name: Run Django checks
+        run: uv run manage.py check
+      - name: Run tests with coverage
+        run: uv run pytest -v --cov bbs --cov-report term --cov-report xml:coverage.xml
+
+      - name: Upload coverage reports
+        uses: actions/upload-artifact@v3
+        with:
+          name: coverage-report
+          path: coverage.xml