CI: skip backend pipeline for frontend-only pull requests

[hotzenplotz5]

Summary

• classify pull request changes before scheduling expensive jobs
• keep full CI for pushes and every PR touching files outside web/frontend/
• skip Make audit, C++ fast regression, daemon build, packaging and install staging for frontend-only PRs
• keep docs and frontend regression checks for frontend-only PRs
• use the Node runtime already present on GitHub-hosted runners instead of running apt-get update and reinstalling Node
• preserve full CI for workflow, build, backend, packaging and mixed changes

Safety model

A PR is considered frontend-only only when every changed file is below web/frontend/. Any other path sets full_ci=true.

Push events always run the full pipeline.

This PR itself changes .github/workflows/ci.yml, so it must and should run the complete pipeline before merge.

Expected effect

Future frontend-only PRs should complete after change classification, docs checks and frontend-regression-test, while the expensive C++/daemon/packaging chain reports skipped.