feat: add gitlab mr stats to pr metrics

[Akanksha-020]

Summary

Brief description of what this PR does.

Closes #10

Type of Change

• Bug fix
• [..] New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• Fetch GitLab MRs when session.gitlabToken exists and merge with GitHub PR stats.
• Return combined totals plus per‑source stats in the PR metrics API.
• Display a GitLab “Sources” section in the PRMetrics card when available.

How to Test

Steps for the reviewer to verify this works:

1. Sign in and ensure session.gitlabToken is set.
2. Open the dashboard → verify PR Metrics shows combined totals and a GitLab section.
3. Call GET /api/metrics/prs and confirm sources.gitlab appears with metrics.
4. Repeat without a GitLab token and confirm GitHub‑only stats.

Screenshots (if UI change)

NA

Checklist

• [.. ] Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• [.. ] Self-reviewed the diff
• Added/updated tests if applicable

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