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/audit_log.rs

@@ -1,10 +1,10 @@
-use mongodb::{
-    Collection,
-    bson::{doc, oid::ObjectId},
-};
-use futures::stream::TryStreamExt;
-use serde::{Deserialize, Serialize};
 use anyhow::Result;
+use futures::stream::TryStreamExt;
+use mongodb::{
+    bson::{doc, oid::ObjectId},
+    Collection,
+};
+use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum AuditEventType {
@@ -87,7 +87,8 @@ impl AuditLogRepository {
     }
 
     pub async fn find_by_user(&self, user_id: &ObjectId) -> Result<Vec<AuditLog>> {
-        let cursor = self.collection
+        let cursor = self
+            .collection
             .find(
                 doc! {
                     "user_id": user_id
@@ -102,15 +103,13 @@ impl AuditLogRepository {
 
     pub async fn find_recent(&self, limit: u64) -> Result<Vec<AuditLog>> {
         use mongodb::options::FindOptions;
-        
+
         let opts = FindOptions::builder()
             .sort(doc! { "timestamp": -1 })
             .limit(limit as i64)
             .build();
 
-        let cursor = self.collection
-            .find(doc! {}, opts)
-            .await?;
+        let cursor = self.collection.find(doc! {}, opts).await?;
 
         let logs: Vec<AuditLog> = cursor.try_collect().await?;
         Ok(logs)