refactor: remove OpenRouter rollback, shims, and redundant layers

- OpenRouter: native Anthropic only; remove chat_request and OPENROUTER_TRANSPORT
- Drop OpenAICompatibleProvider alias, api.request_utils, voice_pipeline facade
- Simplify OpenRouter SSE, generic reasoning in conversion, messaging dispatch
- Shared markdown table helpers; API optimization response helper; contract guards
- Restore PLAN.md; update docs and tests

api/app.py

@@ -13,7 +13,7 @@ from providers.exceptions import ProviderError
 
 from .dependencies import cleanup_provider
 from .routes import router
-from .runtime import AppRuntime, warn_if_process_auth_token
+from .runtime import AppRuntime
 
 # Opt-in to future behavior for python-telegram-bot
 os.environ["PTB_TIMEDELTA"] = "1"
@@ -23,11 +23,6 @@ _settings = get_settings()
 configure_logging(_settings.log_file)
 
 
-def _warn_if_process_auth_token(settings) -> None:
-    """Compatibility wrapper for tests importing the old app helper."""
-    warn_if_process_auth_token(settings)
-
-
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """Application lifespan manager."""