feat(self-managed): add function-autoscaler release

[bright-poku]

TL;DR

Wire the function-autoscaler workload into the self-managed stack's stage-03
observability helmfile, gated behind a new functionAutoscaler toggle that is
disabled by default.

Additional Details

• Adds a function-autoscaler release to
  deploy/stacks/self-managed/helmfile.d/03-observability.yaml.gotmpl,
  conditioned on functionAutoscaler.enabled and ordered after state-metrics.
• Adds the functionAutoscaler values block to environments/base.yaml
  (disabled by default; empty endpoints; chart-version placeholder).
• The release inherits the shared service template. SECRETS_PATH and
  CONFIG come from the chart defaults; only the in-cluster endpoints
  (Cassandra contact points and VictoriaMetrics URL) are environment-specific.
• Depends on the stage-01 Cassandra and OpenBao migrations and a reachable
  VictoriaMetrics from the observability stack.

Draft until the function-autoscaler chart is published: chartVersion is a
0.0.0 placeholder that must be pinned to the published
helm-nvcf-function-autoscaler version before this can merge.

For the Reviewer

• Two files: the base.yaml values block and the 03-observability release.
• Note the release is inert by default (condition: functionAutoscaler.enabled,
  which is false in base), so this is safe to land ahead of enabling any env.

For QA

Validated with helmfile list against the stage-03 helmfile:

• Default (toggle off): function-autoscaler renders ENABLED=false (not actioned).
• --state-values-set functionAutoscaler.enabled=true: renders ENABLED=true,
  namespace nvcf, chart nvcf/helm-nvcf-function-autoscaler, ordered after
  state-metrics.

Issues

Closes #352

Checklist

• I am familiar with the Contributing Guidelines.
• I have signed off my commits for Developer Certificate of Origin (DCO) compliance.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/wire-function-autoscaler-self-managed

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