Add a uuid column to the agents table schema

Add a uuid column to the agents table. The existing varchar(64) primary key is kept as-is.

- Column: uuid, UUID type (GUID), UNIQUE, server_default uuid_generate_v4() following the existing pattern in models/base.py.
- Update the ORM model (AgentRow).
- Alembic migration that adds the column and backfills existing rows.

JIRA Issue: BA-7212

Start by reading the existing UUID pattern in models/base.py and locating the AgentRow ORM model. Then review the relevant Alembic migration structure and verify that the migration adds the unique UUID column, backfills existing agents, and preserves the varchar(64) primary key.

Written by the indexing model from the issue text.