Provider passthrough

Runtime-configurable passthrough endpoint under /p/{provider}/{endpoint}; enabled by default via server.enable_passthrough_routes. The endpoint path is opaque and may proxy JSON, binary, or SSE responses with upstream status codes preserved. For multi-segment provider endpoints, clients that rely on OpenAPI-generated path handling should URL-encode embedded slashes in the endpoint parameter. A leading v1/ segment is normalized away by default so /p/{provider}/v1/... and /p/{provider}/... map to the same upstream path relative to the provider base URL.