Add DeploymentPolicy GQL API

## Overview

Create GraphQL API for DeploymentPolicy (deployment strategy configuration).

**Note:** AutoScalingRule API already exists in `auto_scaling_rule.py`. This issue focuses only on DeploymentPolicy.

## Background

`DeploymentPolicyRow` exists in `src/ai/backend/manager/models/deployment_policy.py` with:

- `strategy`: ROLLING or BLUE_GREEN
- `strategy_spec`: RollingUpdateSpec or BlueGreenSpec (JSONB)
- `rollback_on_failure`: boolean

## Tasks

### 1. Create GraphQL Types

```
enum DeploymentStrategyType {
ROLLING
BLUE_GREEN
}

type RollingUpdateConfig {
maxSurge: Int!
maxUnavailable: Int!
}

type BlueGreenConfig {
previewReplicas: Int!
}

union StrategyConfig = RollingUpdateConfig | BlueGreenConfig

type DeploymentPolicy {
id: ID!
strategy: DeploymentStrategyType!
strategyConfig: StrategyConfig!
rollbackOnFailure: Boolean!
createdAt: DateTime!
updatedAt: DateTime!
}
```

### 2. Implement Query

```
deploymentPolicy(deploymentId: ID!): DeploymentPolicy
```

### 3. Implement Mutations

```
createDeploymentPolicy(input: CreateDeploymentPolicyInput!): CreateDeploymentPolicyPayload
updateDeploymentPolicy(input: UpdateDeploymentPolicyInput!): UpdateDeploymentPolicyPayload
deleteDeploymentPolicy(input: DeleteDeploymentPolicyInput!): DeleteDeploymentPolicyPayload
```

### 4. Input Types

```
input RollingUpdateConfigInput {
maxSurge: Int
maxUnavailable: Int
}

input BlueGreenConfigInput {
previewReplicas: Int
}

input CreateDeploymentPolicyInput {
deploymentId: ID!
strategy: DeploymentStrategyType!
rollingUpdateConfig: RollingUpdateConfigInput
blueGreenConfig: BlueGreenConfigInput
rollbackOnFailure: Boolean
}

input UpdateDeploymentPolicyInput {
deploymentId: ID!
strategy: DeploymentStrategyType
rollingUpdateConfig: RollingUpdateConfigInput
blueGreenConfig: BlueGreenConfigInput
rollbackOnFailure: Boolean
}
```

## File Location

`src/ai/backend/manager/api/gql/model_deployment/deployment_policy.py`

## Acceptance Criteria

- [ ] DeploymentPolicy type with strategy configuration
- [ ] Query for fetching policy by deployment ID
- [ ] CRUD mutations for policy management
- [ ] Unit tests

JIRA Issue: BA-3428