Add maintainer analytics trends

[saurabhhhcodes]

Summary

• add cached maintainer analytics data scoped to the active installation repositories
• surface 12-week merge-rate, XP-distribution, active contributor, and contributor-level trend widgets on the maintainer dashboard
• add pure aggregation helpers with focused coverage for weekly buckets, stale-window exclusion, and monthly level distribution

Fixes #92

Testing

• npm run typecheck ✅
• npm run lint ✅ (passes with pre-existing warning in src/app/(app)/issues/issues-list.tsx)
• prettier --check src/lib/maintainer/analytics.ts src/lib/maintainer/analytics.test.ts src/app/actions/maintainer.ts 'src/app/(app)/maintainer/page.tsx' ✅
• git diff --check ✅
• direct tsx sanity check for buildMaintainerAnalytics ✅

Local environment notes

• npm test -- src/lib/maintainer/analytics.test.ts is blocked locally by the existing Rollup native binary macOS code-signing error (@rollup/rollup-darwin-arm64).
• npm run build is blocked locally by the same native binary code-signing class of issue for @next/swc-darwin-arm64.

GSSoC / NSoC label request

If accepted, could you please add gssoc:approved, level:advanced, quality:clean, type:feature, type:backend, and type:design? This should also be eligible for NSoC if the project is tracking the same issue/PR there.

[vercel]

@saurabhhhcodes is attempting to deploy a commit to the codersogs-3057's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

Hey @saurabhhhcodes

You have 4 open PRs right now. The limit is 3 at a time.

Please get your existing PRs merged or closed before opening new ones:

• Add manual mentor PR verification #140 - Add manual mentor PR verification
• fix(issues): match repo filter case-insensitively #137 - Fix Browse Issues repo filter matching
• feat(xp): add suspicious XP flag audit #136 - Add suspicious XP flag audit

This PR will remain open but won't be reviewed until you're under the limit. See our Contributing Guidelines for details.

[saurabhhhcodes]

Thanks for the heads-up. I’m closing this PR for now to respect the 3-open-PR contributor limit.

The branch is ready and validated locally, so I’ll reopen/resubmit it once one of my existing MergeShip PRs is merged or closed.