test: Add test coverage reporting to CI

[param20h]

🧪 Task

Configure pytest-cov and add a coverage report step to .github/workflows/ci.yml. Target: ≥ 60% coverage on the backend.

Difficulty: 🟡 Medium

[aayushprsingh]

Hi, I am a GSSoC 2026 contributor and would like to work on this. I have experience with TypeScript and can get started right away.

Here's how I'm thinking of approaching this:
I'll start by adding pytest-cov to the project and updating the CI workflow to trigger coverage reporting on every run. From there, I'll audit the current test suite to identify the biggest coverage gaps and add the necessary unit tests to ensure we're hitting at least 60% across the backend.