Add SEO i18n schema acceptance gates - #282

Summary

• add an audit acceptance guard for localized metadata, hreflang, sitemap, language switching, social previews, and schema coverage
• strengthen the JSON-LD build gate to assert representative tool WebApplication fields required by the SEO/schema audit
• preserve existing SEO/i18n implementation while making its acceptance criteria explicit and regression-tested

Validation

• npm test -- tests/guards/seo-i18n-schema-acceptance.test.ts tests/guards/seo-metadata-keywords.test.ts tests/guards/bf-p0-seo-routing.test.ts tests/guards/bf-og-images.test.ts tests/guards/seo-canonical-hreflang-foundation.test.ts
• npm run build:app
• npm run check:canonical
• npm run check:hreflang
• npm run check:metadata-localization
• npm run check:jsonld-structured-data
• npm run validate
• npm run build:post

Closes #238
Closes #239
Closes #264