Update EndpointRow with revision and deployment state

## Overview

Add revision management and deployment state columns to EndpointRow. Ensure existing API compatibility.

## Tasks

1. Add columns to endpoints table: current_revision (GUID), deploying_revision (GUID), revision_history_limit (default 10), deployment_state (default 'stable')
1. Add `DeploymentState` enum (stable, deploying, promoting, rolling_back)
1. Update EndpointRow model with new columns and relationships
1. Update existing code to use current_revision for session creation
1. Migrate data: set current_revision to the created revision (from DeploymentRevisionRow story)
1. Maintain backward compatibility for existing GQL/REST APIs
1. Write unit tests

## Reference

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

JIRA Issue: BA-3423