Apply Field Metadata to API Layer

## Overview

Apply `BackendAIAPIMeta` field metadata annotations to API layer components for consistent documentation and version tracking.

## Scope

Apply metadata to:

- REST API DTOs (`src/ai/backend/manager/dto/*.py`)
- Strawberry GraphQL schema (`src/ai/backend/manager/api/gql/*.py`, `models/gql_models/*.py`)

## References

- [BEP-1022: Pydantic Field Metadata Annotation](https://github.com/lablup/backend.ai/blob/main/proposals/BEP-1022-pydantic-field-annotations.md)
- Depends on: BA-3766 (metadata class implementation)

## Implementation Phases

- Phase 3: Apply to REST API Request/Response DTOs
- Phase 4: Apply to Strawberry GraphQL types and inputs

JIRA Issue: BA-3783