feat: add project scaffold with core client, error types, and models

Set up Cargo.toml with dependencies (reqwest, serde, thiserror, tokio,
uuid, chrono). Implement OnspringClient with builder pattern and HTTP
helper methods. Define error types, enums, and paging models.

src/lib.rs

@@ -0,0 +1,10 @@
+#![allow(dead_code)]
+
+pub mod client;
+mod endpoints;
+pub mod error;
+pub mod models;
+
+pub use client::{OnspringClient, OnspringClientBuilder};
+pub use error::{OnspringError, Result};
+pub use models::*;