Adds UUID FK columns alongside integer FKs across PhoenixKit tables.

Called from V56 migration. This helper module keeps V56 manageable by extracting the UUID FK column creation logic (~80 columns across ~40 tables).

How It Works

For each FK column, three operations:

1. ALTER TABLE ... ADD COLUMN IF NOT EXISTS {uuid_fk} UUID
2. Backfill via JOIN: UPDATE t SET {uuid_fk} = s.uuid FROM source s WHERE s.id = t.{int_fk}
3. CREATE INDEX IF NOT EXISTS ... ON table({uuid_fk})

Large tables use batched backfills (PL/pgSQL DO block, 10k rows/batch) to avoid long-running transactions.

After columns are created and backfilled, add_constraints/1 adds:

• NOT NULL constraints where the integer FK is NOT NULL
• FK constraints where the integer FK has an explicit DB-level FK constraint

Safety

• All operations wrapped in table_exists? + column_exists? checks
• FK constraint creation uses pg_constraint existence check (idempotent)
• NOT NULL uses ALTER COLUMN SET NOT NULL (idempotent in PostgreSQL)
• Idempotent — safe to run multiple times