feat: Add health check endpoint

Introduce a new health check endpoint at `/api/health`. This endpoint
returns a JSON response indicating the server's status and configuration
details, such as the port, data path, and configured URLs for external
services.

This change also refactors the application setup by:
- Creating a `lib.rs` file to house the `create_router` function, making
  it
  reusable for both the main application and integration tests.
- Moving configuration loading logic into `main.rs` and passing it as a
  shared state (`Arc<Config>`) to the router.
- Adding a unit test (`health_test.rs`) to verify the functionality of
  the
  health check endpoint.
- Updating `Cargo.toml` and `Cargo.lock` to include the `tower`
  dependency,
  which is used for testing.

server/src/lib.rs

@@ -0,0 +1,14 @@
+pub mod config;
+pub mod error;
+pub mod routes;
+
+use std::sync::Arc;
+
+use axum::Router;
+
+use config::Config;
+
+/// Build the application router. Used by main.rs and integration tests.
+pub fn create_router(config: Arc<Config>) -> Router {
+    routes::api_router().with_state(config)
+}