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/main.rs

@@ -1,12 +1,18 @@
-mod config;
-mod error;
-
-use config::Config;
 use std::sync::Arc;
 
+use tokio::net::TcpListener;
+
+use doctate_server::config::Config;
+
 #[tokio::main]
 async fn main() {
     dotenvy::dotenv().ok();
     let config = Arc::new(Config::from_env());
-    println!("Server starting on port {}", config.server_port);
+
+    let addr = format!("0.0.0.0:{}", config.server_port);
+    println!("Server starting on {addr}");
+
+    let app = doctate_server::create_router(config);
+    let listener = TcpListener::bind(&addr).await.unwrap();
+    axum::serve(listener, app).await.unwrap();
 }