Top-level group organization backfill process

What does this MR do?

This MR implements an event-driven process for automatically creating organizations and transferring root groups based on the root_group_organization_backfill feature flag.

Backend work for minimal backfill (#603329 - closed)

Building on the feature flag event support merged in !234843 (merged), this creates the complete backfill process that:

• Subscribes to feature flag events
• Creates organizations for root groups on enable
• Transfers root groups to/from organizations
• Handles both individual actors and global operations