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

[vercel]

@Akanksha-020 is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Priyanshu-byte-coder]

This PR conflicts with recently merged changes. Please rebase onto main:

git fetch upstream && git rebase upstream/main

Resolve any conflicts, push, and I'll review.

[Akanksha-020]

okay I will do that and get back to you

[Priyanshu-byte-coder]

Changes Required

1. Depends on unmerged GitLab OAuth (PR #317)
session.gitlabToken won't exist until the GitLab OAuth PR lands — and that PR itself has blocking issues. This feature silently no-ops until then. Please mark as draft or rebase after #317 merges.

2. Rebase required
Conflicts with PRs #392 and #396 which also modify prs/route.ts. Rebase after those are resolved.

3. Only 100 GitLab MRs fetched — no pagination
per_page=100 with no follow-up pages means heavy GitLab users lose data silently. Add X-Total-Pages header handling or at least document the 100-MR cap.

4. Missing withMetricsCache wrapper
Other metric routes (post PR #311) use withMetricsCache. This route hits both GitHub and GitLab on every request with no caching.

5. GitHub state === "closed" counts rejected PRs as merged
githubStats.merged++ triggers on pr.state === "closed" which includes PRs closed without merging. Use pr.pull_request?.merged_at != null to count only actually merged PRs (same fix applied in PR #396).