feat(deploy): add Helm chart and Kubernetes guide

[however-yir]

Problem

Docker Compose is useful for local evaluation, but production-oriented users need a Kubernetes deployment path with explicit configuration and operational defaults.

Scope

• Add a minimal Helm chart for the application and frontend.
• Document external dependencies for MySQL, Redis/RabbitMQ, pgvector, and observability.
• Provide secure configuration guidance for secrets and tenant settings.

Acceptance criteria

• Helm chart renders with default values.
• Kubernetes guide is added under docs/.
• Deployment notes include secrets, probes, resources, and rollback guidance.