feat(caching): implement stale-while-revalidate for GitHub API calls and set Cache-Control for SVG responses

[Rakshak05]

Description

Fixes #7252

This PR implements response caching with a stale-while-revalidate strategy for GitHub contributions API calls:

• Caching Strategy: Contribution data is cached in Redis using a wrapped structure { data, fetchedAt }. Calls with age < 1 minute serve the cached data immediately. Calls with age < 1 hour serve cached data immediately and trigger a background revalidation. Other calls fetch synchronously and update the cache.
• Quota Awareness: Background revalidation is paused if the worst-case remaining token quota drops below 100 points, protecting against rate limit exhaustion.
• Cache-Control Header: Default SVG responses now have a static Cache-Control header of public, max-age=300, stale-while-revalidate=3600.
• Tests & Quality: Updated and successfully ran all unit tests for the GitHub caching layer and API routes.

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Bug fix, refactoring, docs)

Visual Preview

N/A (Backend caching and header updates)

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 started 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.

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[retenta-bot]

This PR enhances the caching strategy for GitHub API calls by implementing a stale-while-revalidate approach, which should improve performance and user experience. It also sets a new Cache-Control header for SVG responses, aligning with previous decisions to ensure timely updates for users. Please refer to the related decision on updating Cache Control Headers for SVG Endpoints for additional context. Thank you for your contribution!

[github-actions]

📦 Next.js Bundle Size Report (Gzipped Sizes)

✨ No significant bundle size changes detected.

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	3985.28 KB	3985.28 KB	0 B
Total CSS	331.41 KB	331.41 KB	0 B

[Aamod007]

This is a massive performance and reliability upgrade! Implementing stale-while-revalidate caching and updating the HTTP cache-control headers for the SVG endpoints ensures blazing fast load times while keeping data fresh in the background. The changes to the caching layer and tests are well executed.

Excellent work on this!

Label Justification:

• \level:critical: Assigned based on >= 4 files changed, fundamentally refactoring the caching architecture and updating all associated tests.
• \quality:clean: Clean background revalidation logic and updated test cases.
• \ ype:performance: Directly improves API response latency via SWR caching.
• \mentor:Aamod007: Assigned as required.

[github-actions]

🎉 Congratulations @Rakshak05! Your PR has been successfully merged. 🚀

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