Expose PostgreSQL migration settings in the Helm chart

[iplay88keys]

Summary

Expose the PostgreSQL schema migration configuration through the fork's Helm chart now that the upstream Goose implementation is included in v0.0.23.

This PR does not change the migration implementation. It adds the Helm integration needed to configure the schema and use an external PostgreSQL database.

Changes

• Add postgres.schema, defaulting to public.
• Pass the configured schema to ate-api-server through ATE_API_POSTGRES_SCHEMA and --postgres-schema=@env.
• Add postgres.enabled so installations using an external database can disable the bundled PostgreSQL StatefulSet.
• Require postgres.connectionString when bundled PostgreSQL is disabled.
• Document the new Helm values.
• Regenerate the checked-in API server environment ConfigMap.
• Fix hack/render-manifests.sh so preserved manifests are correctly excluded during generation and verification.

Upgrade notes

• The upstream migration implementation is already present in fork release v0.0.23.
• Databases created by older development releases contain application tables without a migration ledger and must be recreated before upgrading. In-place adoption of those databases is intentionally not included.
• The published v0.0.23 chart predates these Helm changes, so the chart integration will require a subsequent fork release.