ci: deduplicate workflows using reusable test pipeline and stable tagging

- Introduce reusable-test workflow for Ruff + E2E tests
- Refactor CI to call reusable workflow (no duplicated steps)
- Add stable-tag workflow to promote version tags to stable after successful tests

https://chatgpt.com/share/694a7f81-d96c-800f-88cb-7b25b4cdfe1a

.github/workflows/reusable-test.yml

@@ -0,0 +1,86 @@
+name: Reusable Test (ruff + e2e)
+
+on:
+  workflow_call:
+    inputs:
+      python-version:
+        type: string
+        required: false
+        default: "3.12"
+      matomo-url:
+        type: string
+        required: false
+        default: "http://127.0.0.1:8080"
+      matomo-admin-user:
+        type: string
+        required: false
+        default: "administrator"
+      matomo-admin-password:
+        type: string
+        required: false
+        default: "AdminSecret123!"
+      matomo-admin-email:
+        type: string
+        required: false
+        default: "administrator@example.org"
+      matomo-token-description:
+        type: string
+        required: false
+        default: "ci-token"
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ inputs.python-version }}
+          cache: "pip"
+
+      - name: Install system deps (curl)
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y curl
+
+      - name: Install Python deps
+        run: |
+          python -m pip install --upgrade pip
+          pip install -e ".[e2e]"
+          pip install ruff
+
+      - name: Ruff
+        run: |
+          ruff check .
+          ruff format --check .
+
+      - name: Install Playwright Chromium
+        run: |
+          python -m playwright install --with-deps chromium
+
+      - name: E2E (docker compose + installer + tests)
+        env:
+          MATOMO_URL: ${{ inputs.matomo-url }}
+          MATOMO_ADMIN_USER: ${{ inputs.matomo-admin-user }}
+          MATOMO_ADMIN_PASSWORD: ${{ inputs.matomo-admin-password }}
+          MATOMO_ADMIN_EMAIL: ${{ inputs.matomo-admin-email }}
+          MATOMO_TOKEN_DESCRIPTION: ${{ inputs.matomo-token-description }}
+        run: |
+          make e2e
+
+      - name: Docker logs (on failure)
+        if: failure()
+        run: |
+          docker compose -f tests/e2e/docker-compose.yml ps || true
+          docker compose -f tests/e2e/docker-compose.yml logs --no-color --tail=300 matomo || true
+          docker compose -f tests/e2e/docker-compose.yml logs --no-color --tail=300 db || true
+
+      - name: Cleanup (always)
+        if: always()
+        run: |
+          docker compose -f tests/e2e/docker-compose.yml down -v || true