GraphQL API Metadata and Validation Enhancement

## Overview

Enhance GraphQL API with structured metadata management and unified input validation.

## Scope

### BEP-1031: GraphQL API Field Metadata Extension

- Define `backend_ai_field()`, `backend_ai_type()`, `backend_ai_input()` wrappers
- Integrate `BackendAIAPIMeta` with Strawberry GraphQL
- Enable automatic version prefix in descriptions
- Support deprecation_reason from metadata

### BEP-1032: Unified Input Validation for REST and GraphQL

- Share Pydantic models between REST and GraphQL
- Use `strawberry.experimental.pydantic.input()` for GraphQL inputs
- Centralize validation logic in `data/{domain}/specs.py`

## Goals

- Consistent API metadata management across REST and GraphQL
- Programmatic access to version information for documentation
- Single source of truth for input validation
- Reduced code duplication between API layers

## Related BEPs

- BEP-1022: Pydantic Field Metadata Annotation
- BEP-1031: GraphQL API Field Metadata Extension
- BEP-1032: Unified Input Validation

JIRA Issue: BA-3928