Add organization_id to authentication_events

What does this MR do and why?

This MR adds organization_id as a sharding key to the authentication_events table as part of GitLab's Cells and Organizations initiative.

Background

The authentication_events table currently lacks a proper sharding key, which is required for GitLab's Cells architecture. Investigation in #553447 (closed) revealed that not all authentication events have an associated user_id, making organization_id a more suitable sharding key for this table.

Implementation Details

This is Step 1 of a 3-step migration process:

• Step 1 (This MR): Add organization_id column with default value 1
• Step 2: Update application code to ensure organization is properly set for new records
• Step 3: Remove default value constraint once all records have proper organization associations

Technical Context

• Table Size: Medium (as documented in db/docs/authentication_events.yml)
• Feature Category: System Access
• Schema: gitlab_main_org (organization-level data)
• Model: AuthenticationEvent - tracks user authentication attempts across various providers

References

• Parent Issue: Add organization_id as sharding key for authentication_events table #561359
• Investigation Issue: Sharding Key Investigation for authentication_events table #553447
• Cells Architecture Documentation
• Organization Sharding Guidelines

Database Impact

• Migration Type: Non-blocking (uses concurrent operations)
• Downtime: None expected
• Index Addition: Concurrent index on organization_id
• Foreign Key: Added with cascade delete to maintain referential integrity
• Default Value: Temporary default of 1 (default organization) for existing records