feat(presentation): GET /health endpoint

crates/presentation/src/handlers/health.rs

@@ -0,0 +1,10 @@
+use axum::{extract::State, Json};
+use crate::state::AppState;
+
+pub async fn health_handler(State(s): State<AppState>) -> Json<serde_json::Value> {
+    let db_ok = s.users.list_with_stats().await.is_ok();
+    Json(serde_json::json!({
+        "status": if db_ok { "ok" } else { "degraded" },
+        "db": if db_ok { "connected" } else { "error" },
+    }))
+}