Why: the backend startup layer had only defaults and CLI flags; the
configuration standard's baseline (file, env, CLI layers, read-only explain,
instance isolation) requires the full pipe for dev/test/prod operation.
Evidence: fields and categories mapped one-to-one onto the existing app
Options; runtime settings API kept as the separate business-policy layer.
Impact: design record only; implementation follows in the backend repo.
Refs: standards/configuration.md; backend feature/config-surface
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