Summary

The uuid_and_denormalize migration failed on non-empty databases because it tried to add required columns (user_email, updated_at, user_id) without default values.

Fix

Rewritten as a 4-phase migration:

1. Add all new columns as nullable
2. Backfill denormalized data from joins (emails from users table, timestamps from created_at)
3. Set NOT NULL constraints (safe now that all rows have values)
4. Add foreign keys