test(background-refresh): add explicit timeout for massive scaling tests

[kanishka-2007-tech]

Description

• Added explicit timeout for high-volume scaling tests
• Prevented CI failures caused by Vitest default 5000ms timeout
• Preserved existing test coverage and assertions

Testing

• npm run test services/github/background-refresh.massive-scaling.test.ts
• All tests passing

Fixes #3698

Pillar

• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Bug fix, refactoring, docs)

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).
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• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.
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