feat(self-managed): add highAvailability config schema and global.yaml.gotmpl mapping

Description

Add a single highAvailability: block to the self-managed Helmfile stack and wire it through global.yaml.gotmpl onto chart values (replicaCount, affinity, topology spread, PDB, rolling update, Tier-2 sizing).
Profiles: production | staging | single-node.
Nested keys: tier1, nats, openbao, cassandra.

Definition of Done

• highAvailability schema documented/defaulted in env values (environments/base.yaml and/or production env)
• global.yaml.gotmpl maps highAvailability.* to existing chart value paths
• helmfile build with profile: production renders HA replicas/affinity/PDB
• profile: single-node (or enabled: false) keeps single-replica / soft placement behavior
• No requirement for operators to hand-edit per-service manifests

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