ci: migrate workflows from GitHub Actions to Gitea Actions

- Move CI workflow from .github/workflows/ to .gitea/workflows/
- Add Gitea Actions release workflow for tag builds
- Remove GitHub-specific release workflow

.gitea/workflows/ci.yml

@@ -0,0 +1,37 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  lint-and-test:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ./backend
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+          pip install -r requirements-dev.txt
+
+      - name: Lint with ruff
+        run: ruff check .
+
+      - name: Format check with ruff
+        run: ruff format --check .
+
+      - name: Run tests
+        run: pytest -q