Backfill organization_id for keys table

What does this MR do and why?

This MR implements a batched background migration to backfill the organization_id column in the keys table, supporting the organization-level data model.

Key changes:

• Backfills organization_id for SSH keys from their associated users
• Backfills organization_id for deploy keys from their associated projects (via deploy_keys_projects)
• Assigns orphaned deploy keys (without project associations) to the default organization
• Processes records in batches to minimize database impact

Changelog: changed

References

Fixes: #577245 (closed)

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