🧹 refactor: Extract cache key string formatting to utility functions

[suranig]

🎯 What: Extracted the literal string formatting for stale: and lock: cache keys into centralized utility functions (getStaleKey and getLockKey in src/utils/keys.ts).
💡 Why: Reduces duplication and improves maintainability by standardizing cache key generation. It minimizes the risk of inconsistent key formatting.
✅ Verification: Ran npm run test, npm run format, and npm run lint. All unit tests passed, confirming no functional regression. Reviewed by code reviewer.
✨ Result: Improved code health through consistent and centralized cache key formatting.

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PR created automatically by Jules for task 13077091326882884724 started by @suranig

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[codecov]

Codecov Report

❌ Patch coverage is 98.30508% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/backends/index.ts	94.11%	1 Missing ⚠️

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