refactor!: switch from keydb to valkey due to langfuse incompatibility

[a-klos]

This pull request replaces KeyDB with Valkey as the default in-cluster key-value store for development environments, due to compatibility issues with Langfuse. All documentation, configuration, and code references have been updated accordingly. Additionally, the key-value store implementation now supports authentication via username and password, and related secrets are managed through Kubernetes. The most important changes are grouped below:

Migration from KeyDB to Valkey:

• All references to KeyDB in documentation (README.md, infrastructure/README.md) and configuration files (values.yaml, Chart.yaml) are replaced with Valkey, including feature flags, Helm dependencies, and descriptions. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
• Documentation clarifies that Valkey is now the recommended in-cluster key-value store for development, while managed Redis (or external Valkey) is recommended for production.

Configuration and Secrets Management:

• Kubernetes secrets for Valkey authentication (username and password) are introduced and referenced in the admin backend deployment and values files. [1] [2] [3] [4]
• Admin backend and Langfuse configuration updated to use Valkey hostnames, ports, and authentication settings. [1] [2] [3]

Codebase Refactoring for Key-Value Store:

• The key-value store implementation is moved from impl.key_db to impl.key_value_store in the Python codebase, and all imports are updated accordingly. [1] [2] [3] [4] [5] [6]
• The FileStatusKeyValueStore constructor now supports optional username and password parameters for Redis/Valkey authentication. [1] [2] [3]

Other Documentation and Usability Improvements:

• Various documentation improvements for clarity, including instructions for updating email addresses in cert-manager templates and improved explanations of key-value store usage. [1] [2]

These changes ensure compatibility with Langfuse, improve security via secret management, and clarify the recommended deployment practices for the key-value store.