fix: reorder migration 151 to delete duplicates before update

[bokelley]

Summary

• Fix migration 151 that was blocking deployments due to unique constraint violation
• Reorder SQL operations to delete duplicate records before updating Slack IDs to WorkOS IDs

Problem

Migration 151 (consolidate_slack_workos_ids.sql) was failing with:

duplicate key value violates unique constraint "idx_wg_membership_unique"
Key (working_group_id, workos_user_id)=(29c61a7a-..., user_01KEF3D9...) already exists.

The migration tried to UPDATE Slack IDs to WorkOS IDs before deleting duplicates, causing the unique constraint to fail when both records already existed.

Solution

Reordered operations to:

1. DELETE Slack-ID-based records where a WorkOS-ID-based record already exists
2. UPDATE remaining Slack IDs to WorkOS IDs (now safe - no duplicates possible)
3. Same pattern for working_group_leaders table

Test plan

• All tests pass
• Migration logic reviewed for data integrity
• Migration runner wraps each migration in a transaction (rollback on failure)

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