feat: cache GitHub metrics with Upstash Redis

[saurabhhhcodes]

Summary

• add a reusable Upstash Redis metrics cache helper with graceful no-env fallback
• cache GitHub-backed metrics for contributions, repos, PR summaries, and streak active dates
• apply endpoint-specific TTLs: contributions 5m, repos 10m, PRs 10m, streak 2m
• add deterministic keys scoped by user/account, endpoint, and query params
• support explicit cache bypass/refresh via refresh=1, bypassCache=true, sync=true, or x-devtrack-cache-bypass: 1
• document optional Upstash env vars in .env.example

Fixes #234

GSSoC label request

If accepted, please add:

• gssoc:approved
• level:advanced
• quality:clean
• type:performance
• type:backend

Testing

• node /tmp/codex-npm-cli/bin/npm-cli.js run type-check ✅
• node /tmp/codex-npm-cli/bin/npm-cli.js run lint ✅ passes with pre-existing warnings in BadgeSection.tsx and CommitTimeChart.tsx
• git diff --check ✅
• node -e "JSON.parse(require('fs').readFileSync('package-lock.json','utf8'))" ✅

Build note

• node /tmp/codex-npm-cli/bin/npm-cli.js run build compiles successfully, then fails while collecting page data because this local environment does not have Supabase env configured (supabaseUrl is required) in /api/auth/link-github. That is unrelated to the metrics cache change.

[vercel]

@saurabhhhcodes 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.

[saurabhhhcodes]

Quick maintainer note: the PR is mergeable and the code-side checks passed locally (type-check, lint, git diff --check). Vercel deployment is waiting for team authorization, which I cannot complete from the fork.

If this is accepted for GSSoC, could you please add gssoc:approved, level:advanced, quality:clean, type:performance, and type:backend? This closes the assigned caching issue #234.