Round 19: Trade discount source of truth + marketing stubs + changelog feed

Unified the trade discount: /me/tier and checkout now both read pricing_scheme.relative_adjustment, so what the PDP badge claims is what the cart charges. Shipped /blog, /integrations, /careers, /partners marketing stubs so the apex footer links all resolve. Added this changelog + JSON feed. Seeded Organization and WebSite JSON-LD on the canonical host for richer search snippets.