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

tests/e2e/Dockerfile.e2e

@@ -0,0 +1,17 @@
+FROM python:3.12-slim
+
+WORKDIR /opt/app
+
+# Copy project
+COPY . .
+
+# Install the project (editable is fine for tests)
+RUN python -m pip install -U pip \
+ && python -m pip install -e . \
+ && python -m pip install -U unittest-xml-reporting >/dev/null 2>&1 || true
+
+# Create /Backups in container (our tests will use it)
+RUN mkdir -p /Backups
+
+# Run E2E unittest
+CMD ["python", "-m", "unittest", "-v", "tests.e2e.test_e2e_docker"]