[GEO][P1] Expose post sources as citation in Article JSON-LD #102

Problem

Posts carry traceable sources (relationship to research-sources) but buildArticleJsonLd in src/seo/structuredData.ts does not expose them. Source transparency is a key trust signal for answer engines.

Proposal

• Map populated public sources to schema.org citation (CreativeWork with name, url, publisher when available).
• Skip unpopulated IDs and sources without a public URL; never leak private registry notes.

Acceptance

• Article JSON-LD includes citation when sources exist, omitted otherwise.
• Unit tests in the structured data suite.