Add a UUID unique identifier column to the agents table

The agents table currently uses a varchar(64) agent identifier string (e.g. "i-ubuntu") as its primary key and has no UUID-typed identifier.

Goal: add a "uuid" column to the agents table.

- Type: UUID, with a UNIQUE constraint.
- Server-default generated (uuid_generate_v4()), consistent with the UUID id columns of other tables.
- The existing varchar primary key is kept as-is; this epic only adds the new column (no PK replacement).
- Includes the ORM model change and an Alembic migration that backfills existing rows.

JIRA Issue: BA-7211

Start with the agents ORM model and inspect the UUID id columns of other tables for the established server-default pattern. Add the UUID field and an Alembic migration that backfills existing agents while preserving the varchar primary key; done means the column is UUID-typed, uniquely constrained, and server-generated.

Written by the indexing model from the issue text.