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/routes/health.rs

@@ -0,0 +1,18 @@
+use std::sync::Arc;
+
+use axum::extract::State;
+use axum::Json;
+use serde_json::{json, Value};
+
+use crate::config::Config;
+
+pub async fn handle_health(State(config): State<Arc<Config>>) -> Json<Value> {
+    Json(json!({
+        "status": "ok",
+        "port": config.server_port,
+        "data_path": config.data_path.to_string_lossy(),
+        "users": config.users.iter().map(|u| &u.slug).collect::<Vec<_>>(),
+        "whisper_url": config.whisper_url,
+        "ollama_url": config.ollama_url,
+    }))
+}

server/src/routes/mod.rs

@@ -0,0 +1,11 @@
+mod health;
+
+use std::sync::Arc;
+
+use axum::Router;
+
+use crate::config::Config;
+
+pub fn api_router() -> Router<Arc<Config>> {
+    Router::new().route("/api/health", axum::routing::get(health::handle_health))
+}