Implement DeploymentRevisionRow

## Overview

Create deployment_revisions table and implement SQLAlchemy model with Repository CRUD methods. Migrate existing endpoint deployment settings to revisions.

## Tasks

1. Create `deployment_revisions` table with columns: id, endpoint, revision_number, image, model, model_mount_destination, model_definition_path, model_definition (JSONB), resource_slots, resource_opts, cluster_mode, cluster_size, startup_command, bootstrap_script, environ, callback_url, runtime_variant, extra_mounts, created_at
1. Implement `DeploymentRevisionRow` SQLAlchemy model with relationships
1. Add Repository methods: create_revision, get_revision, list_revisions, get_active_revision
1. Write migration script to create revision_number=1 for each existing endpoint
1. Write unit tests

## Reference

- Design doc: docs/endpoint-model-restructure-TODO.md

JIRA Issue: BA-3420