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.

.gitignore

@@ -0,0 +1,5 @@
+/target
+Cargo.lock
+*.swp
+*.swo
+.env