Add foreign key constraint between user_audit_events and users table for protocells sharding

Problem

The user_audit_events table does not have a foreign key constraint on the users table, as noted in !202547 (diffs).

This will cause issues with the sharding effort for protocells because:

• When organizations are moved to different cells, we will end up with copies of audit events in different cells
• These rows will become orphaned, meaning we lose track of where data for an organization could be
• This impacts data integrity and the ability to properly manage organizational data across cells

Proposal

Add a foreign key constraint between user_audit_events and users table to ensure proper referential integrity and support the protocells sharding effort.

Considerations

• The *_audit_event tables are partitioned monthly but will be retained indefinitely
• This is a medium-term effort that needs to be co-designed with the Org Data migration team
• Need to consider the impact on existing data and migration strategy

Related Issues

• Original sharding key investigation: #553473 (closed)
• Related MR: !202547 (merged)

Action Items

• Design foreign key constraint implementation
• Coordinate with Org Data migration team (@luciezhao @mkozono)
• Plan migration strategy for existing data
• Consider impact on partitioned table structure