docs(express): clarify settlement timing for dynamic routes

[Bortlesboat]

Fixes #1886.

Summary

• clarify in the Express README that payments are verified before the handler runs and settled only after a successful response
• add a concrete dynamic-route example for paid server-side operations
• add the same guidance to the top-level FAQ, including the caveat for irreversible work

Verification

• npx --yes prettier --check typescript/packages/http/express/README.md docs/faq.md
• git diff --check

[vercel]

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