Add pytest-cov integration and establish module-level coverage targets

[johnwmillr]

Problem

We currently lack integrated coverage reporting and clear module-level targets, which makes it hard to track progress and prevent regressions.

Scope

1. Add pytest-cov and coverage config
2. Generate and record baseline coverage
3. Add CI coverage checks
4. Track improvements per module

Acceptance Criteria

• pytest-cov is configured
• uv run pytest --cov=lyricsgenius --cov-report=term-missing --cov-report=html works
• Baseline coverage is documented by module
• CI runs coverage on PRs
• Minimum threshold is enforced (initially modest, then raised over time)

Initial Module Targets

• lyricsgenius/api/ >= 90%
• lyricsgenius/types/ >= 90%
• lyricsgenius/genius.py >= 85%
• lyricsgenius/utils.py >= 80%
• lyricsgenius/auth.py >= 85%

Implementation Plan

Phase 1: Baseline

• Add pytest-cov
• Run baseline coverage and capture gaps by module

Phase 2: Enforcement

• Add CI coverage workflow
• Enforce initial --cov-fail-under threshold
• Document contributor commands in README/CONTRIBUTING

[prajp98]

Hi, I'd like to work on this issue

[johnwmillr]

Hi @prajp98, please do! I can't make any promises that I'll review your work quickly, but I welcome a PR (or series of PRs) that address the acceptance criteria laid out in this issue.