Set up pre-commit hooks for ruff and eslint

[sanmaxdev]

Background

CI runs ruff and eslint, but contributors only find out after pushing. A pre-commit config that runs the linters/formatters locally would catch issues earlier and keep PRs clean.

Tasks

• Add a .pre-commit-config.yaml with hooks for ruff (lint + format) on backend/
• Add a hook (or lint-staged setup) for eslint on frontend/
• Document pre-commit install in CONTRIBUTING.md

Acceptance criteria

• Running the hooks matches what CI enforces
• Setup steps are documented

See CONTRIBUTING.md.