Add backup webhook event guidance

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Summary

• Add backup.succeeded and backup.failed to webhook guidance for shared, Vitess, and Postgres skills.
• Recommend routing backup failures to operations or agent triage, while using successful backup events for recovery audit evidence where needed.
• Update the best-practices matrix and MCP agent loop guidance to prefer webhook-triggered backup failure triage over polling.

Changelog entry

• https://planetscale.com/changelog/backup-webhook-events

Why

PlanetScale added backup success and failure webhook events for both Vitess and Postgres. The skills should now recommend evaluating these event subscriptions as part of backup/PITR posture and safe automation design.