feat: add SEO foundation for Audio Recognition

[icecold009]

SEO foundation

• add author identity, canonical/OG/Twitter metadata, and JSON-LD
• add robots and sitemap discovery for the public deployment where a verified URL exists
• keep private or operational routes out of search where applicable
• preserve the existing product behavior and evidence boundaries

This change is limited to SEO/discovery metadata and does not add secrets, provider calls, or application data changes.