fix: switch health-check email notifications to Postmark SMTP

[beveradb]

Summary

• Swap smtp.sendgrid.net → smtp.postmarkapp.com (port 587, STARTTLS) in all three health-check workflows
• Swap SENDGRID_API_KEY secret → POSTMARK_SERVER_TOKEN (now set at the org level)
• SendGrid account was cancelled mid-Apr 2026, so failure alerts from these workflows have been silently dropping for ~3 weeks

@coderabbitai ignore

Test plan

• CI passes (no tests in this repo, just lint/format if any)
• Manually trigger one of the health workflows (e.g. `gh workflow run gen-health.yml`) — won't send email unless the service is actually down, but should run clean
• Next genuine production health failure → verify alert email arrives via Postmark

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