Introduce an OpenAPI-first HTTP client driven by the same specification

used for server-side route binding, and refactor documentation and
templates to treat the client as a first-class contract consumer.

Key changes:
- Add OpenAPI-first client module based on httpx
- Document client usage alongside server-side binder usage
- Update mkdocs navigation to include client documentation
- Refactor CRUD and model app templates to call APIs via operationId
  instead of hardcoded paths
- Align package documentation and public API surface with client support
- Clarify server/client dependency split (fastapi vs httpx)

This establishes strict symmetry between OpenAPI-driven server binding
and OpenAPI-driven client invocation, reinforcing OpenAPI as the single
source of truth on both sides of the contract.

openapi_first/loader.py

@@ -25,8 +25,6 @@ This module intentionally does NOT:
 - Perform request/response validation at runtime
 """
 
-from __future__ import annotations
-
 import json
 from pathlib import Path
 from typing import Any