ci: add GitLab CI pipeline aligned with pre-commit configuration

Description

Add GitLab CI/CD pipeline configuration to automate linting, testing, and building.

Changes

• Set up 4 pipeline stages: setup, lint, test, build
• Configure jobs matching pre-commit checks:
  • Lint: ruff check and ruff format --check
  • Test: mypy, pytest (with coverage ≥75%), vulture, bandit
  • Build: uv build with artifact preservation
• Enable uv caching based on uv.lock for faster pipeline runs
• Add coverage reporting with Cobertura format
• Run lint/test jobs on merge requests, default branch, or relevant file changes
• Run build job on tags or default branch

Checklist

• Pipeline stages mirror local pre-commit hooks
• UV cache configured for CI performance
• Coverage threshold set to 75%