Bump GitHub Actions, test on Python 3.14, add Dependabot

[nanomad]

• Bump actions/checkout to v7, actions/setup-python to v7, actions/cache to v6 and JRubics/poetry-publish to v2.1.
• Pin pytest-results-action to v0.9.0 instead of main.
• Add Python 3.14 to the test matrix.
• Include the Python version in the venv cache key. Before, all matrix jobs shared one cache key.
• Add .github/dependabot.yml for GitHub Actions and pip, weekly. Dev dependencies are grouped.
• Update the pre-commit hooks: poetry 2.5.1, ruff v0.17.1 (matches the project) and pre-commit-hooks v6.0.0.

The new action majors run on Node 24 and need a runner of at least 2.327.1. GitHub-hosted ubuntu-latest meets that.