add spec-driven auth dependency injection

- security.py: parse securitySchemes, resolve {ENV_VAR} from env,
  generate FastAPI Depends for Bearer JWT introspection
- app.py: extract schemes, build deps, pass to binder at init
- binder.py: inject Depends() per operation based on spec's security
- __init__.py: export security module
- pyproject.toml: add httpx dependency

openapi_first/__init__.py

@@ -102,6 +102,7 @@ from . import client
 from . import errors
 from . import codegen
 from . import codegen_routes
+from . import security
 
 __all__ = [
     "app",
@@ -111,4 +112,5 @@ __all__ = [
     "errors",
     "codegen",
     "codegen_routes",
+    "security",
 ]