Add migration to backfill user.pubid column part 3

h/migrations/versions/9b7171cdeb8a_backfill_user_pubid_part_3.py

@@ -0,0 +1,31 @@
+"""Backfill user.pubid with unique values part 3."""
+
+from alembic import op
+from sqlalchemy.orm import sessionmaker
+
+from h.migrations.versions.f32200e2e496_backfill_user_pubid import backfill_users
+
+revision = "9b7171cdeb8a"
+down_revision = "f29f80083c26"
+
+
+USER_PUBID_LENGTH = 12
+USER_BATCH_LIMIT = 1000
+USER_MAX_COUNT = 1_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