test(ci): add coverage for static-checks

Runs mypy and ruff. Add basic test.

- Update pyproject.toml dependencies groups
  - PDM can use top-level [dependency-groups] (PEP 735), support was added in pdm 2.20.0 (October 2024)
- Fix Ruff warnings

.github/runtime/action.yml

@@ -0,0 +1,39 @@
+---
+name: Install runtime
+description: Install the required runtime and related base dependencies
+
+inputs:
+  python:
+    description: "Install Python"
+    default: "true"
+
+outputs:
+  python:
+    description: "Resolved version"
+    value: ${{ steps.get_version.outputs.python }}
+
+runs:
+  using: "composite"
+  steps:
+    - name: Determine version
+      id: get_version
+      shell: bash -eo pipefail {0}
+      run: |
+        python="$(grep -E '^requires-python\s*=\s*\"' pyproject.toml | sed -E 's/.*\"[^0-9]*([0-9]+\.[0-9]+).*/\1/' | head -n1)"
+        if [ -z "$python" ]; then
+          exit 1
+        fi
+        echo "python=${python}" >> "$GITHUB_OUTPUT"
+
+    - name: Install Python
+      if: ${{ inputs.python == 'true' }}
+      uses: actions/setup-python@v6
+      with:
+        python-version: ${{ steps.get_version.outputs.python }}
+
+    - name: Show tooling information
+      shell: bash -eo pipefail {0}
+      run: |
+        set -x
+
+        python --version

.github/workflows/static-checks.yml

@@ -0,0 +1,67 @@
+name: Static Checks
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  workflow_dispatch:
+
+concurrency:
+  group: static-checks-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  lint-and-typecheck:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Install runtime
+        id: runtime
+        uses: ./.github/runtime
+
+      - name: Set up PDM
+        uses: pdm-project/setup-pdm@v4
+        with:
+          python-version: ${{ steps.runtime.outputs.python }}
+          cache: true
+
+      - name: Install dependencies
+        run: pdm sync -dG:all
+
+      - name: Ruff
+        run: pdm run ruff check src
+
+      - name: Mypy
+        run: pdm run mypy -p mpd_now_playable
+
+  tests:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Install runtime
+        id: runtime
+        uses: ./.github/runtime
+
+      - name: Set up PDM
+        uses: pdm-project/setup-pdm@v4
+        with:
+          python-version: ${{ steps.runtime.outputs.python }}
+          cache: true
+
+      - name: Install dependencies
+        run: pdm sync -dG:all
+
+      - name: Run tests
+        run: pdm run pytest tests