feat(api): add GitHub health check to health endpoint

[adepat06]

Description

Fixes #7806

Adds a lightweight GitHub health check to the existing /api/health endpoint.

Changes

• Added a minimal GitHub GraphQL health check using viewer { login }.
• Returns 503 with githubApi: "error" when the GitHub API is unreachable or the configured token is invalid/expired.
• Returns 200 with githubApi: "ok" when the GitHub health check succeeds.
• Keeps the endpoint independent of the contribution data cache and user-specific requests.
• Added route tests covering both healthy and failure scenarios.

Pillar

• 🛠️ Other (Bug fix, refactoring, docs)

Visual Preview

N/A (API-only change)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• I have updated README.md if I added a new theme or URL parameter.
• I have starred the repo.
• I have made sure that I have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard (no raw elements, smooth animations, correct fonts).
• (Recommended) I joined the CommitPulse Discord community for contributor discussions, mentorship, and faster PR support.

[github-actions]

🚨 Hey @adepat06, the CI Pipeline is failing on this PR and it has been marked as status:blocked.

Please fix the issues before this can be reviewed. Here's how:

1. Run checks locally before pushing:

npm run format:check   # Check Prettier formatting
npm run lint           # Run ESLint
npm run typecheck      # TypeScript type check
npm run test           # Run unit tests (Vitest)
npm run build          # Verify production build passes

2. Auto-fix common issues:

npm run format         # Auto-fix formatting with Prettier
npm run lint -- --fix  # Auto-fix lint errors where possible

3. Check the full failure log here:
👉 View CI Run

Once you push a fix and the CI passes, the status:blocked label will be removed automatically. 💪