Resend webhook verifier + normalizer.
Verifier: Svix-style HMAC-SHA256 over svix-id.svix-timestamp.raw_body.
Resend signs one or more v1,<base64> values in the svix-signature
header, allowing zero-downtime secret rotation. This verifier decodes
the configured whsec_... secret, enforces a replay window, and
compares signatures with Plug.Crypto.secure_compare/2.
Normalizer: decodes the JSON object body and maps Resend's type
strings into the Anymail taxonomy verbatim where possible. Unmapped
event types fall through to :unknown with Logger.warning.