fix: scope S3 realtime events to system managers

[sankarsubramaniankvs]

Summary

• scope S3 realtime events to System Managers only
• reuse a shared helper instead of leaving publish_realtime unscoped
• keep existing event names while restricting recipients

Fixes

• closes [Security] Scope realtime events for S3 operations to authorized users/rooms #9

Changes

• add _publish_system_manager_realtime(...) helper in aws_integration/s3/handlers.py
• route s3_upload_complete through that helper
• route migration, cleanup, and orphan progress/completion events through that helper

Security impact

• prevents cross-user leakage of S3 operational metadata
• ensures these notifications are raised only to System Managers

Validation

• python3 -m compileall aws_integration
• inspected existing scoped realtime pattern in backup flow and aligned with it