Implement DeploymentPolicyRow

## Overview

Create deployment_policies table for deployment strategy configuration (Rolling Update, Blue-Green).

## Tasks

1. Create `deployment_policies` table with columns: id, endpoint (unique), strategy, strategy_config (JSONB), rollback_on_failure, created_at, updated_at
1. Implement `DeploymentPolicyRow` SQLAlchemy model
1. Add `DeploymentStrategy` enum (rolling_update, blue_green)
1. Add pydantic models: `RollingUpdateConfig`, `BlueGreenConfig` for strategy_config validation
1. Add Repository methods: create_policy, get_policy, update_policy
1. Create default rolling_update policy for existing endpoints
1. Write unit tests

## Reference

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

JIRA Issue: BA-3422