Opt-in route 500s if OPTIN_HMAC_SECRET is missing (no graceful handling)

Recommendation

- In the route, guard the token verification with a try/catch that logs a clear server-misconfig event and returns a 500 with a generic error page. - Alternatively, validate OPTIN_HMAC_SECRET at module init of the route (or app startup) and fail fast with a clear boot-time error, preventing the app from serving until the secret is present. - Example: at top-level of route.ts, read process.env.OPTIN_HMAC_SECRET and if missing, log and export a GET that returns a 500 error page with a friendly message (no details about secrets). - Consider adding a healthcheck that asserts required env vars are present.

Recommendation

Either wrap verifyTokenDetailed in try/catch in handleOptIn (returning a generic 500 errorPage), or have verifyTokenDetailed return {kind:"invalid"} when the secret is missing (less safe — masks misconfig).