ci: eliminate duplicate PR runs, add pr-gate

[nikolareljin]

Summary

• `ci.yml`: removed `pull_request` trigger (was firing 5 jobs twice per PR push); added `paths-ignore` with recursive `**/*.md` glob; CI install switched to `npm ci` with lockfile-based cache
• `pr-gate.yml`: new — backend flake8 syntax check + frontend build on every PR; `paths-ignore` intentionally omitted (required status check must not be skippable)
• `frontend/package-lock.json`: added lockfile so `npm ci` works in CI
• `frontend/package.json`: bumped vite 5→6.4.2 and vitest 1→3.x to fix CVEs GHSA-4w7w-66w2-5vf9 (vite path traversal) and GHSA-67mh-4wv8-2f99 (esbuild); `@vitejs/plugin-react` updated to 4.7.0 for vite 6 compatibility
• Full Docker builds and docker-compose validation stay in `ci.yml` (push-only) — no point rebuilding images on every PR commit

Test plan

• Push to main triggers `ci.yml` only (backend lint, frontend build, Docker builds, config validate, security scan)
• PR open/push triggers `pr-gate.yml` only (2 jobs: backend lint, frontend build)
• No duplicate runs on PR pushes
• `npm audit` reports 0 vulnerabilities after vite/vitest upgrade

[Copilot]

Pull request overview

This PR restructures GitHub Actions workflows to avoid duplicate CI runs on PR pushes by moving lightweight PR checks into a dedicated workflow while keeping heavier Docker-related work push-only.

Changes:

• Removed the pull_request trigger from ci.yml and added paths-ignore under push.
• Added a new pr-gate.yml workflow to run backend flake8 syntax checks and a frontend build on PRs.
• Added PR-level concurrency cancellation to reduce redundant in-flight runs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
.github/workflows/pr-gate.yml	New PR-only “gate” workflow that runs lightweight backend linting and a frontend build, with concurrency cancellation.
.github/workflows/ci.yml	Converts CI to push-only and ignores docs/Markdown-only changes to reduce unnecessary runs.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: d2102b65df

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[Copilot]

Pull request overview

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 5 changed files in this pull request and generated no new comments.