Update RoutingRow with revision and traffic status

## Overview

Add revision tracking and traffic status to RoutingRow for deployment management.

## Tasks

1. Add columns to routings table: revision (GUID), traffic_status (default 'active')
1. Add `RouteTrafficStatus` enum (active, standby, draining)
1. Update RoutingRow model with new columns and relationships
1. Update route creation logic to set revision from endpoint.current_revision
1. Migrate data: set revision to endpoint.current_revision for existing routes
1. Update App Proxy routing logic to filter by: status=HEALTHY AND traffic_status=ACTIVE
1. Write unit tests

## Reference

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

JIRA Issue: BA-3424