M10 (post-covenant): Public GET /api/v1/utilities/deprecated endpoint

Replaces the superseded private v_deprecated_utilities view concept. Public endpoint any consumer can poll to detect stale EIA-id linkage keys. Returns: [{eia_id, slug, name, deprecated_at, successor_eia_id, deprecation_reason}]. Filters: since=<iso8601>, state=. Backed by public utilities table columns (deprecated_at, deprecation_reason already exist post-0012; successor_eia_id may need a migration — verify first). Add to public OpenAPI spec. Cache headers: public, s-maxage=3600, stale-while-revalidate=86400. Covenant-compliant by construction — any researcher or developer can use this to track utility lifecycle.