Stale deploy-platform references drift in docs (Fly.io named as API target when stack is actually Railway)

In a monorepo whose API actually deploys to Railway (with Neon Postgres and Vercel frontend), docs and code comments referenced a defunct Fly.io deploy target. Worse: TECH_STACK.md "rejected tools" table claimed "Railway | Replaced by Fly.io" — exactly inverse of reality. Defensive code paths (a Playwright host-guard regex matching *.fly.dev and *.onrender.com) also lingered as dead branches that couldn't fire because the app never runs on those hosts. Cost analyses listed Fly.io as the API line item. Architecture mermaid diagrams labeled the API subgraph as "Fly.io · Hono".