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.

mkdocs.yml

@@ -42,6 +42,7 @@ nav:
       - OpenAPI-First App: openapi_first/app.md
       - Route Binder: openapi_first/binder.md
       - Spec Loaders: openapi_first/loader.md
+      - Client: openapi_first/client.md
 
   - CLI:
       - Home: openapi_first/cli.md