R10 Phase 3: S3 Notification — Event-Driven Webhooks on CAS
Implemented S3 bucket notification configurations with real webhook delivery, prefix/suffix filtering, exponential-backoff retries, and all six S3 event types — so backup tools and pipelines can react to object changes instead of polling. Teaches: why a notification dispatcher that returns nil without sending is the trap to watch for, and how events stay honest across plain PUT, multipart, and delete paths.