Add GitHub Actions CI - ruff + frontend build + pytest

[Dix01]

Problem: PRs are not checked automatically. CI protects main and signals an active, quality project.

Where: new .github/workflows/ci.yml.

Approach: On pull_request + push to main:

• Python: install backend deps, run ruff lint, run pytest
• Frontend: cd web && npm ci && npm run build
• Cache pip + npm

Acceptance:

• CI runs on every PR
• Lint/build/test failures block the merge button

Difficulty: help wanted. Pairs with the test-suite issue.