Add Helm chart for Kubernetes deployment

[treblereel]

Context

Part of epic #1 — Build production-ready REST microservice with k8s deployment on top of casehub-engine. Helm chart simplifies deployment, configuration management, and upgrades across different environments (dev, staging, production).

What

Create Helm chart for casehub-flow deployment:

• Chart structure with templates for Deployment, Service, ConfigMap, Secret, Ingress
• Values files for different environments (dev, prod)
• Parameterized configuration (replicas, resources, database settings, image tag)
• PostgreSQL dependency (bitnami/postgresql subchart)
• Installation and upgrade documentation

Acceptance Criteria

• Helm chart structure created in helm/casehub-flow/ directory
• Templates: Deployment, Service, ConfigMap, Secret, Ingress (optional)
• values.yaml with sensible defaults (replicas: 2, resource limits, health check settings)
• values-dev.yaml and values-prod.yaml for environment-specific overrides
• Image tag, repository, and pull policy configurable via values
• PostgreSQL subchart dependency declared in Chart.yaml (bitnami/postgresql)
• Database connection auto-configured from PostgreSQL subchart values
• ConfigMap template uses environment variables from issue Add production configuration profile with environment variables #9
• Health probe paths configurable (default: /q/health/live, /q/health/ready)
• Chart installs successfully: helm install casehub-flow ./helm/casehub-flow
• Chart upgrades work without downtime (rolling update strategy)
• README with installation instructions and configuration examples

Notes

• Use Helm 3 (no Tiller dependency)
• PostgreSQL dependency using bitnami/postgresql chart (required, not optional)
• Database credentials auto-generated by PostgreSQL subchart or overridable in values
• Consider adding NOTES.txt with post-installation instructions
• Ingress template should be optional (enabled: false by default)
• Chart version should track application version
• File paths: helm/casehub-flow/Chart.yaml, helm/casehub-flow/values.yaml, helm/casehub-flow/templates/*