feat: pyproject-based packaging, unified CI and Docker e2e tests

- migrate to pyproject.toml and pip installation
- introduce cleanback CLI entrypoint
- add unit and Docker-based end-to-end tests
- unify GitHub Actions CI and stable tagging
- remove legacy tests.yml and pkgmgr requirements

https://chatgpt.com/share/69517d20-f850-800f-b6ff-6b983247888f

.github/workflows/ci.yml

@@ -0,0 +1,65 @@
+name: CI
+
+on:
+  push:
+    branches: ["**"]
+    tags:
+      - "v*"
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: Tests (unit + e2e)
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.12"
+
+      - name: Upgrade pip
+        run: python -m pip install -U pip
+
+      - name: Install project (editable)
+        run: python -m pip install -e .
+
+      - name: Run tests
+        run: make test
+
+  tag-stable:
+    name: Tag stable on version tag
+    runs-on: ubuntu-latest
+    needs: [test]
+    if: startsWith(github.ref, 'refs/tags/v')
+
+    permissions:
+      contents: write
+
+    steps:
+      - name: Checkout (full history for tags)
+        uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Configure git user
+        run: |
+          git config user.name  "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+
+      - name: Force-update stable tag to this commit
+        env:
+          SHA: ${{ github.sha }}
+        run: |
+          git tag -f stable "${SHA}"
+          git push -f origin stable