feat(seo): add BlogPosting structured data and article social metadata

[KevinVandy]

Context

Published TanStack blog posts expose titles, descriptions, authors, publication dates, and images in visible SSR content, but they currently emit no Article or BlogPosting JSON-LD. The shared SEO helper also gives every page og:type=website, including blog posts.

Google documentation: https://developers.google.com/search/docs/appearance/structured-data/article

Relevant route: src/routes/blog.$.tsx

Proposed scope

• Emit BlogPosting JSON-LD on every published internal blog post.
• Include accurate values for:
  • headline
  • description
  • image
  • datePublished
  • dateModified when known
  • author as Person or Organization
  • publisher referencing the canonical TanStack Organization node
  • mainEntityOfPage
• Add optional updated/dateModified frontmatter support so revised posts can distinguish publication and modification dates.
• Change blog post social metadata to og:type=article.
• Add article:published_time and article:modified_time when applicable.
• Reuse the canonical social image transformation already used by the route.

Acceptance criteria

• BlogPosting JSON-LD is included in the raw SSR response for published internal posts.
• Draft, future-dated, missing, and error pages do not emit indexable BlogPosting markup.
• Author names use the same normalization as visible bylines.
• Author URLs are included only when they identify a real maintainer profile or another stable public identity.
• Publisher uses the same Organization @id and identity fields as the homepage graph.
• dateModified is accurate; when no modification date is known, it is omitted or intentionally equals datePublished rather than invented.
• Image URLs are absolute, crawlable, and representative of the article.
• og:type is article on post pages and remains website on non-article routes.
• Automated tests cover single author, multiple authors, missing header image, drafts, and modification dates.
• Representative posts pass Google Rich Results Test validation.