Switch cache infrastructure from Azure Cache for Redis to Azure Managed Redis

[akhan-msft]

Summary

The docs guidance already references "Azure Managed Redis" by name in the stateless services section and in the architecture diagram description. This issue ensures the Terraform infrastructure and application configuration match that intent — i.e., actually provision Azure Managed Redis rather than Azure Cache for Redis.

What needs to change

• Terraform
• Application code / configuration

Acceptance criteria

• Terraform provisions azurerm_redis_enterprise_cluster + azurerm_redis_enterprise_database.
• Private endpoint and DNS zone reference the correct AMR sub-resource and zone.
• App connects to Azure Managed Redis and runs end-to-end after azd up.
• README and connection string documentation updated.