feat(helm): improve chart flexibility with HTTPRoute, external DB, and existingSecret support

[KarolayneAmabile]

Summary

• Add support for Gateway API HTTPRoute as an alternative to Ingress (per component)
• Add support for external MongoDB and Redis (bundled databases remain the default)
• Add secrets.existingSecret to reference a pre-existing Kubernetes Secret instead of creating one
• Add _helpers.tpl with auto-derivation of DB_CONNECTION_STRING, CLIENT_HOST, and UPTIME_APP_API_BASE_URL
• Add server.apiBaseURL override for local/port-forward access (no Ingress required)
• Add prechecks.yaml with pre-flight validations (JWT, protocol consistency, TLS requirements, etc.)
• Fix nginx ConfigMap: hardcoded hostname and wrong api-docs proxy target
• Fix MongoDB/Redis StatefulSets: missing serviceName field
• Bump chart version to 0.2.0
• Update INSTALLATION.md to reflect new options and upgrade path from v0.1.x

Backward compatibility

Existing deployments using persistence.mongo.* / persistence.redis.* continue to work without changes. Default behavior (Ingress enabled, bundled MongoDB) is preserved.

Test plan

• helm template renders correctly for all combinations (Ingress, HTTPRoute, external DB, existingSecret)
• helm install with Ingress disabled + port-forward (local mode)
• helm install with Ingress enabled