docs: add Redis testing guide and update documentation references

[oyinade247]

closes #187

PR Description: Document Redis Testing Patterns

Description

This PR adds comprehensive documentation for Redis usage and testing patterns in the TalentTrust Backend. It clarifies the hybrid testing approach where Redis is mocked by default but utilized in CI and for specific integration tests.

Changes

• New Guide: Created docs/backend/redis-testing-guide.md covering:
  • Global mocking strategy in src/test-setup.ts.
  • Scenarios where real Redis is required (Health probes, manual integration).
  • CI/CD service container configuration.
  • Local reproduction steps using Docker and the CI=true flag.
• Queue System Docs: Updated docs/backend/queue-system.md to link to the new guide.
• README: Added discoverability link for the Redis Testing Guide.

Motivation

Previously, the distinction between mocked Redis and real Redis requirements was not explicitly documented, making it difficult for new contributors to understand why certain tests were being ignored locally or how to debug queue-related issues.

Testing Steps

1. Verify the new file docs/backend/redis-testing-guide.md exists and is correctly formatted.
2. Verify links from README.md and docs/backend/queue-system.md are functional.
3. (Optional) Run CI=true npx jest src/queue/queue-manager.test.ts with a local Redis instance to verify the local reproduction instructions.

Checklist

• Documentation updated
• Links verified
• Local reproduction steps validated against jest.config.js logic

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