fix: add service annotations support to metaflow-ui services

[bafulton]

The metaflow-service subchart already supports service.annotations (service.yaml:7-10), but the metaflow-ui subchart does not. This makes it impossible to set per-service annotations on the UI backend and static frontend services.

This is needed for deployments using the AWS Load Balancer Controller, where per-service alb.ingress.kubernetes.io/healthcheck-path annotations are required to configure different healthcheck paths per target group (the backend responds on /api/ping, the static frontend on /). Without service-level annotation support, the only workaround is vendoring the chart or creating duplicate Service resources.

This PR adds {{- with .Values.uiBackend.service.annotations }} and {{- with .Values.uiStatic.service.annotations }} blocks to backend_service.yaml and static_service.yaml, following the exact pattern already used in metaflow-service.

Example usage:

metaflow-ui:
  uiBackend:
    service:
      annotations:
        alb.ingress.kubernetes.io/healthcheck-path: /api/ping
  uiStatic:
    service:
      annotations:
        alb.ingress.kubernetes.io/healthcheck-path: /

Backward compatible — no annotations are rendered when none are configured.

[bafulton]

@josephsirak or @nschmeller, any chance you could take a look? Sorry, I'm not sure who to ping.