Add migration to backfill user.pubid column part 4

h/migrations/versions/01c9594fb9d5_backfill_user_pubid_part_4.py

@@ -0,0 +1,31 @@
+"""Backfill user.pubid with unique values part 4."""
+
+from alembic import op
+from sqlalchemy.orm import sessionmaker
+
+from h.migrations.versions.f32200e2e496_backfill_user_pubid import backfill_users
+
+
+revision = "01c9594fb9d5"
+down_revision = "9b7171cdeb8a"
+
+USER_PUBID_LENGTH = 12
+USER_BATCH_LIMIT = 1000
+USER_MAX_COUNT = 3_000_000
+USER_PUBID_RETRIES = 5
+
+
+def upgrade():
+    session = sessionmaker()(bind=op.get_bind())
+
+    backfill_users(
+        session,
+        USER_BATCH_LIMIT,
+        USER_MAX_COUNT,
+        USER_PUBID_RETRIES,
+        USER_PUBID_LENGTH,
+    )
+
+
+def downgrade():
+    pass