Follow-up to #6013. The signing path silently fell back to using the raw
secret string as the HMAC key when a whsec_-prefixed secret had invalid
base64, producing signatures no receiver could verify with no server-side
signal.
- Extract resolveSigningKey helper that errors on invalid whsec_ base64
- Post returns that error (logged by the async dispatcher); ValidateSigningSecret
rejects it at write time so a bad secret is never stored
- Fix stale comment referencing a nonexistent Authorization header
- Add Go tests: key derivation, secret validation, end-to-end signature
round-trip, and the invariant that the secret never leaks into API responses
