Caught by TESTING the alert path instead of assuming it. Without an explicit
User-Agent, urllib sends 'Python-urllib/3.x' and Resend rejects it 403, while
the identical request via curl succeeds.
The failure mode this avoids is the worst one a canary has: it would have
detected every outage correctly and told nobody. Same bot-filtering trap as the
Gitea migrate call earlier today — worth recognising on sight.
Also prints the HTTP body on failure. '403 Forbidden' alone sends you hunting
for a bad key; the body names the real cause.
Verified: alert sent (200).
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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