i18n SEO pass: sitemap alternates, JSON-LD inLanguage, GSC verification

[spizeck]

International SEO completion/verification pass, per docs/MULTILINGUAL.md §6 (#134, PRs #149/#154). Runs after localized content exists — depends on #150 (foundation) and #151 (Dutch Phase 1 pages).

Boundary with #150

#150 builds the locale-aware metadata plumbing (canonical, alternates.languages seam, og:locale) as infrastructure. This issue owns the SEO completion and verification — nothing here is duplicated acceptance criteria.

Scope

• app/sitemap.ts: emit alternates.languages per entry for localized routes only; no lastModified regression.
• Full reciprocal hreflang verification on every localized page, including x-default → English unprefixed URL.
• JSON-LD: inLanguage on BreadcrumbList/FAQPage/Course on localized pages; single business @id — no per-locale duplicate entities.
• Localized written titles/descriptions and og:locale (nl_NL) verified end-to-end.
• public/llms.txt: add the one-line Dutch-availability note; still points at canonical English URLs. No llms.nl.txt.
• Final localized SEO validation + Search Console international/hreflang coverage notes for the owner.

Acceptance

• Reciprocal hreflang verified on every localized page including x-default.
• Sitemap emits correct alternates; no duplicate/conflicting entities.
• English pages' metadata and URLs unchanged.