Vercel `nodejs` runtime passes IncomingMessage/ServerResponse with
shouldAddHelpers=true (auto-parsed JSON body, .status/.send helpers),
not the Web Standards Request/Response. Calling `req.headers.get(...)`
on the classic IncomingMessage threw `TypeError: req.headers.get is
not a function` and crashed every webhook + cron invocation with 500.
Switch both handlers to (req, res) signature, read headers as plain
object (lowercased keys), use req.body for parsed JSON, and respond
via res.status().send().
Caught during Phase 6 smoke test of the first prod deploy.