Add function-autoscaler release to the self-managed stack

[bright-poku]

Description

The self-managed stack's stage-03 observability helmfile does not yet deploy
the function-autoscaler workload. Add a function-autoscaler release, gated
behind a functionAutoscaler toggle that is disabled by default, so an
environment can opt in to running the autoscaler alongside the observability
stack.

The release inherits the shared service template and is ordered after
state-metrics. It depends on the stage-01 Cassandra and OpenBao migrations
and a reachable VictoriaMetrics from the observability stack. SECRETS_PATH
and CONFIG come from the chart defaults; only the in-cluster endpoints
(Cassandra contact points and VictoriaMetrics URL) are environment-specific.

Definition of Done

• functionAutoscaler values block added to environments/base.yaml
  (disabled by default; empty endpoints; chart version placeholder)
• function-autoscaler release added to
  helmfile.d/03-observability.yaml.gotmpl, conditioned on
  functionAutoscaler.enabled and ordered after state-metrics
• helmfile list renders the release ENABLED=false by default and
  ENABLED=true when the toggle is set
• Chart version pinned once the function-autoscaler chart is published

Resources (optional)

• Wires the workload into the self-managed deploy stack (stage 03 observability).