Registers HandleAlexaCertificate on POST /alexa/certificate. The handler
logs the device MAC from the request body and returns 501 Not
Implemented with a JSON error explaining that AWS IoT integration is
required to provision Alexa device certificates.
Adds voice.api.bose.io to both /etc/hosts domain lists in setup.go (DNS
intercept was already covered by the bose.io wildcard entry in dns.go).
Relates to https://github.com/gesellix/Bose-SoundTouch/discussions/84
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>