feat: add apps endpoint with list, get, and batch-get methods

Implement App model and three endpoint methods: list_apps (paginated),
get_app (by ID), and batch_get_apps (up to 100 by IDs). Add
comprehensive tests for success, pagination, not-found, and batch
scenarios.

src/models/app.rs

@@ -0,0 +1,10 @@
+use serde::Deserialize;
+
+/// Represents an Onspring application.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct App {
+    pub href: Option<String>,
+    pub id: i32,
+    pub name: Option<String>,
+}