Add ElastiCache cluster mode and update docs

[LinaBell]

Summary

• Add ElastiCache cluster mode support for Redis connections
• Update README with comprehensive documentation (features, implementation requirements, environment variables)
• Update .env-example to match active environment variables with safe placeholder values

Test plan

• Verify ElastiCache cluster mode connection works correctly
• Verify standalone Redis connection still works
• Review README for accuracy
• Confirm .env-example contains all required variables

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