M9: Public DB-SCHEMA.md + docs/api-integration.md

Refresh public DB-SCHEMA.md to reflect current utilities table (38+ cols) and ALL new public tables from post-covenant migrations (utility_resolver_cache, utility_name_manual_overrides, substations). Add new docs/api-integration.md describing the full public consumer surface: list+filter + bulk eia_ids + by-eia-id detail route + POST /api/v1/utilities/resolve + territory GeoJSON + PMTiles + edge caching + rate-limit tiers. Covenant-pure: every consumer (including Texture) uses the same public API-key model.