[DevOps]: Upload coverage to Codecov and add a badge (follow-up to #24)

[shauryagangrade]

Problem

CI already runs coverage run -m pytest and uploads .coverage artifacts
(ci.yml), but no one aggregates or publishes the numbers, and #24 asks for
visible coverage tracking.

Proposed approach

• Emit coverage.xml (via coverage xml) in CI and upload to Codecov (or
  Coveralls) on main + PR runs.
• Add the Codecov badge to the README next to the build badge.

Where to look

• .github/workflows/ci.yml:55-68 (test job, coverage run + artifact upload)
• pyproject.toml:66-71 (coverage config)
• README.md:3 (badges)

Acceptance criteria

• Coverage is visible on Codecov with a PR status check.
• README shows the badge.

Difficulty

Low.