style: apply rustfmt to backend codebase

- Apply rustfmt to all Rust source files in backend/
- Fix trailing whitespace inconsistencies
- Standardize formatting across handlers, models, and services
- Improve code readability with consistent formatting

These changes are purely stylistic and do not affect functionality.
All CI checks now pass with proper formatting.

backend/src/models/health_stats.rs

@@ -1,7 +1,10 @@
-use mongodb::Collection;
-use serde::{Deserialize, Serialize};
-use mongodb::{bson::{oid::ObjectId, doc}, error::Error as MongoError};
 use futures::stream::TryStreamExt;
+use mongodb::Collection;
+use mongodb::{
+    bson::{doc, oid::ObjectId},
+    error::Error as MongoError,
+};
+use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct HealthStatistic {
@@ -46,7 +49,11 @@ impl HealthStatisticsRepository {
         self.collection.find_one(filter, None).await
     }
 
-    pub async fn update(&self, id: &ObjectId, stat: &HealthStatistic) -> Result<Option<HealthStatistic>, MongoError> {
+    pub async fn update(
+        &self,
+        id: &ObjectId,
+        stat: &HealthStatistic,
+    ) -> Result<Option<HealthStatistic>, MongoError> {
         let filter = doc! { "_id": id };
         self.collection.replace_one(filter, stat, None).await?;
         Ok(Some(stat.clone()))